Add blog post generation lock for cron automation

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Summary

Implements a cross-agent lock so the 15-minute blog-post generation cron skips when another run is already active.

Workflow

1. Read lock — npm run blog:lock:status
2. Skip if active — exit when artifacts/blog-post-generation.lock.json is active and not expired
3. Acquire lock — npm run blog:lock:acquire (commits + pushes lock file)
4. Continue making blog posts — generation work + deploy
5. Release lock on stop — npm run blog:lock:release

Changes

• scripts/lib/blog-post-generation-lock.mjs — lock read/write, git sync, stale TTL (2h)
• scripts/blog-post-generation.mjs — CLI (status, acquire, release, run)
• AGENTS.md — documents the cron automation sequence
• package.json — blog:lock:* npm scripts

Testing

• status → idle when no lock
• acquire → writes lock with agent metadata and pushes
• second acquire → exit 2 (skip)
• release → removes lock and pushes
• npm run build + local smoke tests pass

Firebase deploy was not run in this cloud environment (no firebase login credentials).