fix(skills): discover Grok skill and command roots

[qinghuanandejiangshi]

Problem and solution

Grok skills under .grok/skills were invisible to the Skills library and slash picker. This adds the project and user roots, reuses the existing GROK_HOME resolver, and adds Grok to the desktop source filter. Native command markdown is discovered too; commands remain passthrough. Skills marked user-invocable: false remain manageable in the library but stay out of the Grok slash picker.

Checks

• Regression test failed before the fix (zero Grok roots), then passed with default, empty and custom GROK_HOME, project precedence, native commands, disable/re-enable and non-invocable skills.
• Workspace Rust tests: 976 passed, 39 ignored. cargo check passed. Both used --features gpui_platform/runtime_shaders because this Mac lacks the Metal compiler.
• Workspace Rust formatting, bun run protocol:check, and client typecheck passed; client tests: 23 passed.
• Actual native UI: the fixture appeared under Grok, the Grok filter worked, and disabling/re-enabling updated the displayed state and skill file.

Limitations and follow-up

No authenticated Grok session or live model request was used; slash discovery and submission were verified by automated tests, not a live session UI. The normal development bundle failed in the local Swift toolchain (swift-package missing a BuildServerProtocol symbol). UI validation used the watcher with a temporary signed UI-only bundle, omitting computer-use/updater helpers. The packaging script was restored and is not part of this PR. No wire types changed.

Related issue

Fixes #170

AI disclosure

OpenAI Codex implemented the change, wrote and ran tests, inspected the native UI, and drafted this description at the submitter's request. This description is AI-generated; it is not represented as independently human-written.

Checklist

• I understand the entire change and take responsibility for the code and all actions taken.
• I wrote this description myself, without LLM-generated text, and will write PR comments myself.
• I disclosed all AI usage, including the tools and extent of involvement.