feat: add built-in /code-review workflow command

[gr3enarr0w]

Summary

Implements upstream #32 by adding a built-in /code-review workflow command.

• adds generateCodeReviewWorkflow() with 7 parallel review angles, verification, and final report phases
• registers /code-review alongside the existing built-in workflow commands
• defaults an empty command argument to the current directory
• handles missing/empty gather output without throwing
• adds parser and command registration coverage

Closes #32.

Review notes

Full scoped review completed before publishing. One issue was found and fixed before PR creation: the gather phase now handles null/missing output instead of dereferencing gathered.diff.

Verification

• npx biome check src/code-review.ts src/builtin-commands.ts tests/builtin-commands.test.ts tests/builtin-workflows.test.ts
• npm run build
• npm run test:unit -- tests/builtin-workflows.test.ts (project script ran full unit suite: 745 passing, 0 failing)

[QuintinShaw]

Thanks for this. There are two /code-review PRs open on #32 (this and #46), so I'm consolidating on one. I'm keeping #46's generator — it does real small/medium/big tier routing, dedups candidates, and takes gh pr diff <n> as input.

The part worth keeping here is the tests: #46 ships none and breaks the existing commands.length === 4 assertion. If you can port your builtin-workflows.test.ts coverage and the 4 -> 5 bump onto #46, I'll close this in its favor.

One heads-up: the target arg here is only interpolated into the gather prompt as a label — the git commands are hardcoded, so the path / PR-number modes don't actually scope the diff.

[gr3enarr0w]

Superseded by #46, which includes this PR's test coverage (builtin-workflows.test.ts, 4→5 count bump) along with auto-resume and the scoped code-review generator.