Serve llms.txt at the site root

## Problem

`https://docs.fallout.build/llms.txt` returns 404. An LLM answering a question about Fallout has to crawl the rendered site and spend most of its tokens on navigation chrome, or fall back on what it learned about NUKE and get the namespaces and the tool name wrong.

[llms.txt](https://llmstxt.org/) is the convention that fixes this: one markdown file at the site root, listing every page with a one-line description.

## Outcome

`https://docs.fallout.build/llms.txt` serves the generated index.

The file itself is **already generated** in the content repo. Fallout-build/Fallout#660 adds a `GenerateLlmsTxt` target that builds `docs/llms.txt` from the frontmatter of `docs/website/`, commits it, and gates it in CI so a page added without regenerating fails the build. That half is in Fallout-build/Fallout#661.

This repo only has to serve it. `static/` is already served at the root here (`/img/social-card.png` resolves), so the whole change is getting `docs/llms.txt` from the content repo into `static/llms.txt` during the site build.

## Acceptance criteria

- [ ] `curl https://docs.fallout.build/llms.txt` returns 200 with the file's content.
- [ ] The served file matches `docs/llms.txt` in Fallout-build/Fallout at the commit the site was built from. Copy it during the build rather than committing a second copy here, so the two cannot drift.
- [ ] It is served as `text/plain` or `text/markdown`, not downloaded as an attachment.

## Notes

Neither half is user-visible alone: this repo without Fallout-build/Fallout#661 has nothing to serve, and Fallout-build/Fallout#661 without this repo generates a file nobody can fetch.

`llms-full.txt` (the whole corpus inlined rather than linked) is a deliberate non-goal for now. Worth doing later, and it is a different file with a different size budget.