Add `/diff` and `/review` — Review session changes UI

[PureWeen]

Summary

The CLI has /diff and /review commands to review what the agent has modified during a session. PolyPilot has no dedicated UI for reviewing session changes.

CLI Behavior

• /diff — show file changes made during the session
• /review — run the built-in code-review agent on session changes

Proposed Implementation

1. "Review Changes" button in session toolbar
2. Diff viewer — show modified files with syntax-highlighted diffs
3. "Run Code Review" action — trigger the built-in review agent on changes
4. File tree — collapsible tree of all files modified in the session
5. Revert individual files — selective undo via /undo or rewind

Priority: 🟢 LOW