feat(harness): add GitHub Copilot CLI harness bundle

harnesses/copilot/Dockerfile

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+# syntax=docker/dockerfile:1
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+ARG BASE_IMAGE
+FROM ${BASE_IMAGE}
+
+ARG COPILOT_CLI_VERSION=latest
+
+RUN mkdir -p /home/scion/.copilot /home/scion/.copilot/skills \
+             /home/scion/.github && \
+    chown -R scion:scion /home/scion/.copilot /home/scion/.github
+
+# Install copilot binary (Alpine/musl variant for scion-base).
+# The binary is a self-contained Node.js SEA — no npm/node dependency needed.
+RUN ARCH=$(uname -m | sed 's/x86_64/x64/;s/aarch64/arm64/') && \
+    if [ "$COPILOT_CLI_VERSION" = "latest" ]; then \
+      URL="https://github.com/github/copilot-cli/releases/latest/download/copilot-linuxmusl-${ARCH}.tar.gz"; \
+    else \
+      URL="https://github.com/github/copilot-cli/releases/download/v${COPILOT_CLI_VERSION}/copilot-linuxmusl-${ARCH}.tar.gz"; \
+    fi && \
+    curl -fsSL "$URL" | tar xz -C /usr/local/bin && \
+    chmod +x /usr/local/bin/copilot
+
+CMD ["copilot"]

harnesses/copilot/README.md

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+# Copilot Harness Bundle
+
+Scion harness bundle for the [GitHub Copilot CLI](https://github.com/github/copilot-cli)
+(`copilot` from `github/copilot-cli`).
+
+## Bundle Layout
+
+```
+harnesses/copilot/
+  config.yaml           # Harness configuration
+  provision.py          # Container-side provisioner (pre-start hook)
+  capture_auth.py       # Post-login credential capture
+  Dockerfile            # Image build (FROM scion-base)
+  cloudbuild.yaml       # Cloud Build configuration
+  README.md             # This file
+  home/
+    .bashrc             # Shell initialization
+    .copilot/
+      settings.json     # Default settings (auto-update off)
+```
+
+## Installation
+
+```bash
+scion harness-config install harnesses/copilot
+```
+
+## Authentication
+
+The Copilot CLI requires a GitHub account with an active Copilot subscription.
+
+### Fine-Grained PAT (Recommended)
+
+Create a [fine-grained personal access token](https://github.com/settings/personal-access-tokens)
+with the **"Copilot Requests"** permission enabled. The token must be user-owned
+(not organization-owned).
+
+```bash
+scion start --harness copilot --env COPILOT_GITHUB_TOKEN=github_pat_...
+```
+
+Token precedence: `COPILOT_GITHUB_TOKEN` > `GH_TOKEN` > `GITHUB_TOKEN`.
+
+**Note:** Classic PATs (`ghp_...`) are not supported by the Copilot CLI.
+
+### Interactive Login (No-Auth Fallback)
+
+If no token is provided, the agent drops to a shell. Run `copilot login` to
+authenticate via browser-based OAuth device flow, then capture credentials:
+
+```bash
+python3 /home/scion/.scion/harness/capture_auth.py
+```
+
+## Known Limitations
+
+- **No turn/model-call limits** — Copilot CLI has no hook dialect for individual
+  turn or model call events. Only `max_duration` (via Scion's external timeout)
+  is supported.
+- **No telemetry integration** — Copilot's OpenTelemetry configuration surface
+  is undocumented.
+- **System prompt is approximate** — system prompt content is prepended to
+  `.github/copilot-instructions.md`; there is no native `--system-prompt` flag.
+- **No project-scoped MCP** — project-scoped MCP server entries are demoted to
+  global scope.
+- **Subscription required** — an active GitHub Copilot subscription is required.
+  The harness provisions successfully without one, but the CLI will fail with an
+  auth error after launch.

harnesses/copilot/capture_auth.py

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+#!/usr/bin/env python3
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Copilot capture-auth script.
+
+Scans for credential files on disk and stores them as project-scoped secrets
+via `sciontool secret set`. Designed to run after the user authenticates
+interactively inside a no-auth agent container (e.g. after `copilot login`).
+
+Reads credential mappings from inputs/capture-auth-config.json (derived from
+the harness config.yaml's auth.types.*.required_files declarations). This
+avoids hardcoding paths or key names in the script.
+
+Exit codes:
+  0 = at least one credential captured
+  1 = error
+  2 = no credentials found (not an error, but nothing was stored)
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import os
+import subprocess
+import sys
+from typing import Any
+
+EXIT_OK = 0
+EXIT_ERROR = 1
+EXIT_NO_CREDS = 2
+
+HARNESS_BUNDLE = os.path.join(
+    os.environ.get("HOME") or os.path.expanduser("~"),
+    ".scion", "harness",
+)
+
+
+def _expand(path: str) -> str:
+    return os.path.expanduser(os.path.expandvars(path))
+
+
+def _load_config(bundle: str) -> list[dict[str, Any]]:
+    config_path = os.path.join(bundle, "inputs", "capture-auth-config.json")
+    if not os.path.isfile(config_path):
+        return []
+    with open(config_path, "r", encoding="utf-8") as f:
+        try:
+            data = json.load(f)
+        except (json.JSONDecodeError, OSError):
+            return []
+    creds = data.get("credentials")
+    if not isinstance(creds, list):
+        return []
+    return creds
+
+
+def _capture_one(
+    entry: dict[str, Any], force: bool
+) -> tuple[bool, str | None]:
+    """Attempt to capture a single credential. Returns (success, error_msg)."""
+    key = entry.get("key", "")
+    source = _expand(entry.get("source", ""))
+    secret_type = entry.get("type", "file")
+    target = entry.get("target", "")
+
+    if not key or not source:
+        return False, "invalid entry: missing key or source"
+
+    if not os.path.isfile(source):
+        return False, None
+
+    cmd = [
+        "sciontool", "secret", "set", key, f"@{source}",
+        "--type", secret_type,
+        "--target", target,
+    ]
+    if force:
+        cmd.append("--force")
+
+    try:
+        result = subprocess.run(
+            cmd,
+            capture_output=True,
+            text=True,
+            timeout=30,
+        )
+    except FileNotFoundError:
+        return False, "sciontool not found in PATH"
+    except subprocess.TimeoutExpired:
+        return False, f"sciontool timed out for key {key}"
+
+    if result.returncode != 0:
+        stderr = result.stderr.strip()
+        if "already exists" in stderr.lower():
+            return True, None
+        return False, f"sciontool failed for {key}: {stderr}"
+
+    return True, None
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(
+        description="Capture auth credentials and store as project secrets"
+    )
+    parser.add_argument(
+        "--force",
+        action="store_true",
+        help="Overwrite existing secrets",
+    )
+    parser.add_argument(
+        "--bundle",
+        default=HARNESS_BUNDLE,
+        help="Path to harness bundle directory",
+    )
+    args = parser.parse_args()
+
+    entries = _load_config(args.bundle)
+    if not entries:
+        print(
+            "capture-auth: no credential mappings found in "
+            "inputs/capture-auth-config.json",
+            file=sys.stderr,
+        )
+        return EXIT_NO_CREDS
+
+    captured = 0
+    errors = 0
+
+    for entry in entries:
+        key = entry.get("key", "<unknown>")
+        source = entry.get("source", "")
+        expanded = _expand(source) if source else ""
+
+        if not expanded or not os.path.isfile(expanded):
+            print(f"capture-auth: {key}: source not found ({source})")
+            continue
+
+        ok, err = _capture_one(entry, args.force)
+        if err:
+            print(f"capture-auth: {key}: {err}", file=sys.stderr)
+            errors += 1
+        elif ok:
+            print(f"capture-auth: {key}: captured from {source}")
+            captured += 1
+
+    if errors > 0 and captured == 0:
+        return EXIT_ERROR
+
+    if captured == 0:
+        print("capture-auth: no credentials found to capture")
+        return EXIT_NO_CREDS
+
+    print(f"capture-auth: {captured} credential(s) captured successfully")
+    return EXIT_OK
+
+
+if __name__ == "__main__":
+    sys.exit(main())

harnesses/copilot/cloudbuild.yaml

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+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Per-bundle Cloud Build configuration for the Copilot harness image.
+# Builds scion-copilot on top of scion-base:<_TAG>.
+steps:
+  - name: 'gcr.io/cloud-builders/docker'
+    id: 'setup-buildx'
+    args: ['buildx', 'create', '--name', 'mybuilder', '--use']
+    env:
+      - 'DOCKER_CLI_EXPERIMENTAL=enabled'
+
+  - name: 'gcr.io/cloud-builders/docker'
+    id: 'bootstrap-buildx'
+    args: ['buildx', 'inspect', '--bootstrap']
+    env:
+      - 'DOCKER_CLI_EXPERIMENTAL=enabled'
+
+  - name: 'gcr.io/cloud-builders/docker'
+    id: 'build-scion-copilot'
+    args:
+      - 'buildx'
+      - 'build'
+      - '--platform'
+      - 'linux/amd64,linux/arm64'
+      - '--build-arg'
+      - 'BASE_IMAGE=$_REGISTRY/scion-base:$_TAG'
+      - '-t'
+      - '$_REGISTRY/scion-copilot:$_SHORT_SHA'
+      - '-t'
+      - '$_REGISTRY/scion-copilot:$_TAG'
+      - '-f'
+      - 'Dockerfile'
+      - '--pull'
+      - '--push'
+      - '.'
+    env:
+      - 'DOCKER_CLI_EXPERIMENTAL=enabled'
+
+substitutions:
+  _REGISTRY: 'us-central1-docker.pkg.dev/${PROJECT_ID}/public-docker'
+  _TAG: 'latest'
+options:
+  dynamicSubstitutions: true
+  machineType: 'E2_HIGHCPU_8'
+timeout: 1200s