feat(code-review): allow per-PR opt-in via label or comment trigger

Currently Code Review runs automatically on every PR open or push (depending on the trigger setting). For teams with mixed PR types — quick dependency bumps, WIP drafts, internal tooling changes — this creates noise and unnecessary cost on reviews where automated analysis isn't needed.

Request: Add a way to trigger Code Review on a per-PR basis, e.g.:

• A GitHub label (e.g. claude-review) that opts a PR in
• A PR comment command (e.g. /claude-review) that triggers it on demand

This would let teams keep the default trigger off and selectively request reviews on PRs where they want the analysis, without changing org-wide settings.

Reference: https://code.claude.com/docs/en/code-review