Implement Worktree-scoped Commit and outcome recovery

[codeacme17]

Tracks #1.

Summary

Implement Worktree-scoped Commit Drafts and safe staged-only Commit with hooks/signing preservation and state-based outcome reconciliation.

Scope

• Maintain one multiline Commit Draft per Repository/Worktree ID in backend session memory.
• Preserve drafts across navigation, refresh, Branch switch, and failure; clear only after verified success or explicit Clear message.
• Enable Commit only with staged content and display exact Worktree path, Branch/Detached HEAD, and staged-file count.
• Support Initial Commit.
• Require prominent confirmation for Detached HEAD Commit.
• Block ordinary Commit on Conflict, In-progress Git Operation, missing identity, unresolved Index lock, or empty Index.
• Revalidate Worktree identity, HEAD OID, and Index tree OID immediately before execution.
• Pass the message through stdin/file-descriptor input; preserve existing hooks and configured signing.
• Reconcile fresh HEAD/Index/status after success, failure, timeout, or cancellation; prevent duplicate retry while outcome is unknown.
• Keep unstaged content and selected Worktree intact after success.

Acceptance criteria

• Commit includes exactly the selected Worktree's staged content and no unstaged content.
• Local Branch, Initial, and confirmed Detached HEAD Commit paths behave as specified.
• Missing identity, hook rejection, signing failure, external Index lock, stale HEAD/Index, timeout, and ambiguous exit preserve the draft and classify recovery correctly.
• Confirmed success reports short SHA/summary and clears only that Worktree's draft.
• Two different Worktrees may Commit concurrently without crossing Index/HEAD boundaries.
• Raw diagnostics are sanitized and no hook/signing behavior is bypassed.

Out of scope

• Amend, allow-empty, sign-off controls, AI-generated messages, signing setup, or committing unstaged content.

Dependencies

• Implement versioned refresh, operation lanes, and reconciliation #7 and Implement safe file-level Stage and Unstage #10.

PRD coverage

• FR5; AC-09, AC-10, AC-19, and AC-21.

Verification

• Real Git tests for Local/Initial/Detached commits, hook/signing stubs, lock and stale races, process interruption, and multi-Worktree concurrency.