Add git diff review and commit prep panel

[slashdevcorpse]

Summary\n- add a local git review API with grouped staged, unstaged, untracked, and deleted file status\n- render a local changes panel with selectable files, inline diff preview, patch copy, commit-message copy, and explicit staging\n- keep the flow local-only; no push, PR creation, reset, or checkout actions are exposed\n- cover git status grouping and selected staging behavior\n\n## Testing\n- pnpm -C apps/codex-claw lint\n- pnpm -C apps/codex-claw build\n- pnpm -C apps/codex-claw test\n\nCloses #86

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Summary by cubic

Add a local git diff review and commit prep panel in Chat via a new local API. You can preview diffs, copy a combined patch or draft commit message, and stage selected files—kept strictly local; aligns with Linear #86.

• New Features
  • Server: adds /api/git-review (GET returns grouped status, per-file diffs, combined patch, draft commit message; POST { action: 'stage', paths } stages selected files).
  • UI: new git-branch toggle in the chat header opens a panel with grouped files, checkboxes to select, inline diff preview, and buttons to copy Patch/Message or Stage selected.
  • Behavior: supports staged/unstaged/untracked/deleted, synthesizes diffs for untracked text files, caps diff size for safety, and includes tests covering grouping and staging.

^{Written for commit 7dedce7. Summary will update on new commits.}

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