feat: add debug extension for programmatic process start

[378-kaiabot]

Note

Automated pull request by Pi, model aperture/hf:zai-org/GLM-5.2. Implemented and validated empirically against
aliou/pi-processes on branch fix/intentional-stop-context-notification. See the local run log for the full investigation.

Summary

Starting a background process during debugging required prompting the agent to use the process tool — there was no
way to do it programmatically from a slash command. This adds a processes-debug extension (not listed in
package.json) that registers /debug:ps:start <name> <command>, which starts a process through the core extension's
new COMMAND_START protocol channel. The process is fully visible to the agent's process tool (list, output, stop).

Verified empirically: loaded the local extensions via pi -ne -e ~/pi-processes-debug/ -e ~/pi-processes-debug/extensions/processes-debug/index.ts,
ran /debug:ps:start test ./tests/e2e/scripts/continuous-output.sh, confirmed the process appeared in the status bar
(ps: ● test), then prompted the agent to list processes — it found test (proc_1b8d) running.

Changes

• extensions/shared/protocol/channels.ts — added COMMAND_START channel alongside existing COMMAND_KILL / COMMAND_CLEAR.
• extensions/shared/protocol/commands.ts — added CommandStartPayload and CommandStartResult types.
• extensions/shared/protocol/index.ts — exported the new types.
• extensions/processes/handlers/commands.ts — handle COMMAND_START: calls manager.start() and notifications.register() with a default empty notify config, same path the process tool's executeStart takes.
• extensions/processes/client.ts — added requestStart client helper that emits on the COMMAND_START channel and returns a promise.
• extensions/processes-debug/index.ts — the debug extension: a single /debug:ps:start command that parses <name> <command> and calls requestStart. Not in package.json; loaded explicitly via -e.

Verification

• tsc --noEmit ✅
• biome check ✅ (10 files, no fixes applied)
• vitest run ✅ (576 passed, 59 test files)

Stacked on #87 — merge that first.

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Run host: golden-hugo-lefevre · Session: 01a00fb1-0d78-790a-8fbf-f6abd049c168 · Model: aperture/hf:zai-org/GLM-5.2