Check changes (branch review)

Slash: /check-changes

1. Read references/mcp-rules.md for MCP transport rules and branch checklist.
2. whoami once → org_slug from whoami.orgs; repo short name from git remote.
3. prepare_review_diff(org_slug, repo) → diff_budget_bytes, git_argv, git_argv_full.
4. Local git at repo root (exec argv list, not shell string).
5. check_changes(org_slug, repo, diff=<full unified diff>, intent=...).
6. Reply from user_guidance only. Branch review is incomplete until step 5 returns.