Generate an LLM-friendly version of the handbook (llms.txt)

[github-actions]

Summary

Adds an LLM-friendly version of the handbook per the llms.txt standard, closing #85. The VitePress build now emits /llms.txt (a link index of all pages) and /llms-full.txt (every page concatenated), served at the site root on GitHub Pages.

Change

• doc/.vitepress/config.mts — register vitepress-plugin-llms by passing it through withMermaid: withMermaid({ vite: { plugins: [llmstxt()] } }). withMermaid merges it with mermaid's own vite plugins, so no manual vite-config splicing is needed.
• package.json / pnpm-lock.yaml — add the vitepress-plugin-llms devDependency.

Verification

• pnpm docs:build on VitePress 2.0.0-alpha.18 generates both files (llms.txt ~295 tokens / 15 links; llms-full.txt ~35K tokens / 15 pages).
• pnpm docs:preview serves them at /llms.txt and /llms-full.txt (real files, not the SPA fallback).
• Ponytail-reviewed — no over-engineering; withMermaid does the plugin merge.

Notes & follow-up

• Why this plugin, and its limit (backed by deep research): vitepress-plugin-llms is renderer-coupled to VitePress, so it won't carry over to the Astro POC (POC: move handbook off VitePress to a headless CMS with reader RBAC + Google Workspace auth #68 / POC (#68): git-backed handbook on Astro + Cloudflare — GitHub auth, review workflow, no CMS/DB #72). Both the current and POC stacks build from the same doc/*.md, so the transition-proof path (a platform-agnostic generator, or an Astro endpoint reading the content collection) is filed as an advisory on POC (#68): git-backed handbook on Astro + Cloudflare — GitHub auth, review workflow, no CMS/DB #72.
• [[TOC]] left as-is: it surfaces in the llms output as escaped \[\[TOC]]. Verified this is standard remark re-serialization (not a bug); an LLM reads \[ as an escaped literal [, so it's understood correctly — not worth a build-time strip.

Closes #85.