Implement safe file-level Stage and Unstage

[codeacme17]

Tracks #1.

Summary

Implement file-level and selected-Worktree group Stage/Unstage with stale-content rejection and partial-success reporting.

Scope

• Add file actions plus Stage all for Changes/Untracked and Unstage all for Staged Changes.
• Reject staging Conflict entries in the MVP.
• Resolve opaque File IDs server-side and pass literal NUL-delimited paths through stdin; never interpolate paths through a shell.
• Re-read file identity, status, and expected diff baseline immediately before execution.
• Implement Initial-Commit-safe Unstage that changes only the selected Worktree Index and never the Working Tree.
• Run through the selected Worktree's local mutation lane.
• Reconcile the Index/status after every attempt and report bulk Partial Success per path.
• Follow a successful file into its new group/baseline when it still exists.

Acceptance criteria

• Only the explicitly selected Worktree Index changes, including when another Worktree contains the same relative path.
• Unstage leaves Working Tree bytes unchanged.
• External file/Index mutation between review and submission returns a stale rejection and current state.
• Conflict paths cannot be staged.
• Bulk success/failure is reported per path without rollback claims.
• Spaces, Unicode, leading dashes, newlines, renames, deletions, and untracked paths are handled safely.
• Independent Stage/Unstage commands in different Worktrees may run concurrently.

Out of scope

• Hunk/line staging, conflict resolution, discard, stash, reset, or clean.

Dependencies

• Implement versioned refresh, operation lanes, and reconciliation #7 and Implement truthful change classification and diff review #9.

PRD coverage

• FR4; AC-07, AC-08, and the Index portion of AC-19/21.

Verification

• Real multi-Worktree integration tests, stale-baseline races, partial group failure fixtures, and Working Tree byte-preservation assertions.