Feature: Implement DecisionHandoff — AI auto-fixes flagged issues before commit #81

Feature: Implement DecisionHandoff (Agentic Fix Loop)

Discovery

While reading the source code, I found that internal/decisionflow/decisionflow.go
already defines DecisionHandoff (5) with the description "handoff to AI agent" —
but it appears unimplemented in the current UI and workflow.

Proposed Behavior

When the user selects "Handoff to AI" after a review:

1. AI receives the flagged issues + original diff
2. AI generates fixes for each flagged line
3. Fixes are applied to the working tree automatically
4. Changes are re-staged
5. A fresh review cycle runs automatically
6. User approves the final result and commits

Why This Is the Killer Feature

Today's workflow: review → manually fix → re-review → commit (slow)
With Handoff: review → one click → AI fixes → approve → commit (fast)

This turns git-lrc from a passive reviewer into an active coding assistant
right at the commit boundary — the ideal moment since context is fresh.

Suggested UX

[R] Re-review   [C] Commit   [S] Skip   [V] Vouch   [H] Handoff to AI ← new

Implementation Notes

• Handoff should only be allowed in PhaseReviewComplete (already enforced by ActionAllowedInPhase)
• Fixes should be applied as a new staged amendment, not auto-committed
• User must explicitly approve before the actual commit proceeds