Prompt 4 — Comments

Implement comments:

• GET /api/articles/:slug/comments
• POST /api/articles/:slug/comments (auth required)
• DELETE /api/articles/:slug/comments/:id (auth required, author only)

Write tests for all endpoints.

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Before committing: run the Verification Protocol (see CLAUDE.md § Verification Protocol). All 7 steps must pass. Do not commit a partial green state.