Add autonomous round-robin site agent (hourly, free…

Details

A free, hands-off background agent that improves the site on a schedule:
- .github/workflows/agent.yml: hourly GitHub Actions cron (free, CPU-only;
  the model runs remotely), single-concurrency, opens a PR each run.
- Drives open-source Llama 3.3 70B served free by Cerebras (~1M tokens/day,
  OpenAI-compatible) via aider, which edits files, runs 'npm run build', and
  auto-fixes until it passes.
- Round-robins three tasks by run_number % 3: (1) implement a new website
  feature, (2) add/improve a site-quality metric, (3) implement a robustness
  improvement from recent web/UX best practices. Prompts in .agent/prompts/.
- Every run opens a PR for human review; nothing reaches main without approval.
- .agent/README.md documents setup (CEREBRAS_API_KEY secret + enabling
  Actions-creates-PRs), limits, safety guardrails, and how to pause/tune.
No application code changed; npm run build is unaffected.

Files changed (5)