Add change-level review UI for ask-before-edits

[anvita-g]

Summary

This resolves issue #27 by adding a structured edit diff in ask-before-edits mode before applying file changes.

• Introduces selective review for multi-change edits via per-change toggles and Apply Selected.
• Keeps the existing fast path with Allow All for one-click full approval.
• Refactors shared diff logic into reusable utilities used by both tool output and permission review UI.

Why this change

• Improves reviewability by showing exactly what will change before approval.
• Reduces risk from all-or-nothing approvals by enabling partial acceptance of edits, especially for large refactors.
• Preserves speed for users who prefer immediate full approval.

Testing

• pnpm build
• pnpm exec tsc --noEmit
• Manual verification completed

Screenshots (if UI changes)

Contributor Recognition

• Preferred public name (optional): Anvita Gollu
• Preferred link (optional):
• OK to feature in CONTRIBUTORS.md: [ ] Yes

Checklist

• Scope is focused
• Docs updated if needed
• No secrets or private data added