Support Cloudflare cron triggers for scheduled agents

[astrobot-houston]

Originally proposed by @kyleboas in #157. Their description is reproduced verbatim below.

All community-submitted pull requests are automatically converted to issues (bugs) & discussions (feature requests, enhancements) where they can be triaged and prioritized. Once prioritized, a PR implementation is created automatically.

Adds a dedicated assistant Flue agent that monitors security RSS/Atom feeds, alerts via webhook, and asks the coding agent to remediate affected repos with PRs.

Also wires Cloudflare cron triggers for Flue agents declaring triggers.cron so the monitor runs on a schedule.

Verification:

• CI=true pnpm --filter @flue/runtime run build
• CI=true pnpm --filter @flue/cli run build
• CI=true pnpm --filter @flue/cli run check:types
• node packages/cli/dist/flue.js build --root examples/assistant --output /tmp/flue-assistant-build --target cloudflare

Build output verified that the generated Cloudflare worker includes scheduled(...) / runCronAgents(...) and that wrangler.jsonc includes triggers.crons for the security monitor schedule.

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Original implementation from #157 by @kyleboas