Make the autonomous site agent manual-only

Details

The hourly schedule had run 34x in ~2 days and opened zero PRs (the free
Llama 3.3 model rarely emits a usable diff), burning a runner every hour for
no output. Drop the cron and keep workflow_dispatch only, with a 'task' input
to pick a specific task or round-robin on 'auto'. Switch to 'npm ci' and update
.agent/README.md to describe the manual-only flow.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A9nJYCSwtVUrxubiU4VAMN

Files changed (2)