[User Story] Add Claude Code automated PR review workflow

[noahwhite]

Story Summary

As a developer, I want Claude Code to automatically review pull requests in ghost-stack, so that I get AI-assisted code review on every PR without manual intervention.

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✅ Acceptance Criteria

• .github/workflows/claude-review.yml exists and is active on main
• Claude posts a review response on PRs when triggered via @claude comment
• @claude mentions in PR comments trigger a response

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📝 Additional Context

• Uses CLAUDE_CODE_OAUTH_TOKEN secret (1-year token from claude setup-token)
• Action: anthropics/claude-code-action@v1
• Trigger: @claude comment from noahwhite only (restricted to prevent abuse on public repo)
• Verified working on PR chore(deps): update mysql docker tag to v9 #287

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📦 Definition of Ready

• Acceptance criteria defined
• No unresolved external dependencies
• Story is estimated
• Team has necessary skills and access
• Priority is clear
• Business value understood

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✅ Definition of Done

• All acceptance criteria met
• Unit/integration tests written & passing
• Peer-reviewed
• Docs updated (if applicable)
• Verified in staging (if needed)
• No critical bugs/regressions

[linear]

GHO-114