feat(git): add repo root selection for nested workspaces

[jonasjancarik]

Summary

• add per-workspace git root selection with bounded repo scan + manual picker
• wire backend root resolution + list_git_roots command
• update git pane UI (path label, change button, active highlight)

Use case

When a workspace points at a parent folder without a .git, the Git pane currently errors. This adds a scoped, opt-in way to target a nested repo (or scan for candidates) so Git status/diffs/logs work without adding each sub-repo as a separate workspace.

Transparency

Vibecoded with Codex; human confirmed working.

Testing

• npm run lint
• npm run typecheck

[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Nice work!

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