fix(git): model non-git workspace availability

[oxwen11]

Summary

• Port of pie#59: git.branch returns repository | not-repository | workspace-unavailable instead of swallowing every probe failure as { current: null }.
• Session composer labels those states. Chat stays usable for a non-git folder; send is blocked when the workspace path is gone. Real git execution failures stay GIT_FAILED.
• Stacked on fix(chat): keep the composer branch footer from jumping #294 (pie refactor(desktop): simplify state management with derived state pattern #55). Draft worktree / Files / Review surfaces from the pie PR are not here — vibest does not have them.

Test plan

• turbo run test --filter=@vibest/server --force -- test/git.test.ts test/rpc-git.test.ts
• Session whose cwd is not a git repo: footer says Not a Git repository, composer still accepts input
• Session whose cwd is a git repo: footer still shows the branch name
• Deleted / unreadable project folder: footer says Workspace unavailable, send stays disabled

Made with Cursor

[github-actions]

React Doctor found 1 issue in 1 file · 1 warning · score 92 / 100 (Great) · full project

1 warning

src/features/chat/runtime/client-prompt-queue.ts

• ⚠️ L0 unused-file

_{Reviewed by React Doctor for commit fb55a36. See inline comments for fixes.}