feat: implement Worktree-scoped Commit and recovery

[Traviinam]

Summary

• add revisioned backend Commit Drafts scoped to each Repository Worktree
• implement staged-only Commit with Initial and confirmed Detached HEAD support
• preserve native Git hooks and signing through an owned Index transaction
• reconcile success, known failure, cancellation, timeout, and Unknown Outcome safely
• expose exact Commit targets, per-Worktree operation state, cancellation, and recovery in the UI

Acceptance criteria

• Commit includes exactly the selected Worktree Index while preserving unstaged content
• drafts survive navigation, Refresh, Branch switch, failure, timeout, and uncertain outcomes
• verified success reports the short object ID and summary and clears only the unchanged submitted draft
• Conflict, In-progress Git Operation, missing identity, stale HEAD or Index, external Index lock, and empty Index are blocked or classified safely
• different Worktrees may Commit concurrently without crossing HEAD, Index, draft, or result state
• diagnostics remain sanitized and hooks or signing are not bypassed

Verification

• npm test: 47 files, 360 tests passed
• npm run lint
• npm run typecheck
• npm run format:check
• npm run build
• git diff --check
• final Standards and Spec reviews found no actionable P1 or P2 issues

Dependency state

• depends on Implement versioned refresh, operation lanes, and reconciliation #7 and Implement safe file-level Stage and Unstage #10, both merged into dev
• targets dev and contains one implementation commit

Known limitations and out of scope

• silent hook failures without trustworthy Git evidence are reported as sanitized process_failed
• ownership-uncertain Index locks are retained for explicit recovery rather than removed
• amend, allow-empty, sign-off, AI-generated messages, signing setup, and committing unstaged content remain out of scope
• the complete MVP release gate remains tracked by Enforce the MVP acceptance, performance, accessibility, and security gate #16

Closes #11