feat: add code-review-checklist skill for code review guidance

[AbhijitK20]

Summary

Adds a single code-review-checklist skill to the initial agents directory. This is a recreation of #903 (auto-closed by history rewrite), addressing the feedback from that PR.

What changed from #903

• Single skill — only code-review-checklist (the original bundled 3 unrelated skills)
• Codebuff-specific — references actual repo conventions from AGENTS.md, known import cycles, free session invariants, and key files to watch
• Not generic boilerplate — includes actionable checks like "no circular dependencies (especially project-files → auth → logger → project-files)" and "bun:test, not jest"

Skill content

The checklist covers:

1. Correctness — edge cases, async handling, side effects
2. TypeScript & Code Style — typing, import patterns, circular deps
3. Conventions — bun runtime, DI over mocking, tmux for CLI tests
4. Security — no secrets in commits, auth token handling, safe file ops
5. Testing — bun:test, mock isolation, error path coverage
6. Freebuff-Specific — IS_FREEBUFF guards, session state transitions, one-instance-per-account
7. Key Files — auth.ts, freebuff-session-store.ts, use-freebuff-session.ts, etc.

Location

common/src/templates/initial-agents-dir/skills/code-review-checklist/SKILL.md

Follows the existing example-skill convention with YAML frontmatter matching the SKILL.md schema documented in the skills README.

Links to original PR: #903