Add installable Flue skill for agent-facing documentation

[astrobot-houston]

Originally proposed by @dcramer in #270. 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.

This adds a proof-of-concept installable Flue skill under skills/flue/. The idea is to make the docs consumable by agents as a curated router plus flat reference files, rather than bundling the whole docs tree or asking an agent to rediscover the same terminology and implementation boundaries every time.

This was generated with Codex using Sentry's skill-writer skill from the getsentry/skills repo. It may not be the final shape you want, but I figured it could be useful as a concrete starting point for agent-facing Flue documentation.

---

Original implementation from #270 by @dcramer