feat(docs): vitepress site for docs/, mermaid rendering, ci build workflow

Implementation of ADR-0022.

* `vitepress`, `vitepress-plugin-mermaid` and `mermaid` added to
  workspace devDependencies. Build settled at ~9 s for the full
  site, sub-second incremental in dev.
* `docs/.vitepress/config.mts`:
  * `srcExclude` drops `docs/README.md` (git/IDE-only index) and
    `docs/decisions/template.md` (authoring scaffold).
  * `rewrites` maps `decisions/README.md` → `decisions/index.md`
    so `/decisions/` resolves to the curated landing while the
    source stays git-friendly.
  * `ignoreDeadLinks` skips cross-repo refs (`../CLAUDE`,
    `../../apps/**`, `../../notes/**`), `localhost:*` dev URLs,
    and the deliberately-excluded `README`/`template` references —
    the source stays usable from a git/IDE reader without
    breaking the site build.
  * Auto-generated sub-sidebar for `/decisions` walks
    `00NN-*.md` so adding an ADR is one file.
  * Mermaid via `withMermaid` with `securityLevel: 'strict'`.
* `docs/index.md` — VitePress Hero landing with four feature
  cards (Architecture, Decisions, Development, Onboarding).
* `package.json` exposes `docs:dev`, `docs:build`, `docs:preview`.
* `docs/development.md` (line ~5): `[setup/](setup/)` → first
  page, since folder-style links don't resolve cleanly under
  `cleanUrls`. Line 330: wrap `${{ github.* }}` in
  `<code v-pre>…</code>` so VitePress's Vue template parser
  doesn't trip on the curly-brace interpolation.
* `.gitea/workflows/docs-site.yml` — builds on every PR (gate
  on parse errors + genuine dead links), uploads the dist as an
  artifact on push to main. The Mermaid plugin regression fence
  greps for `class="mermaid"` / `<svg>` in ADR-0009's rendered
  HTML.
* `.gitignore` excludes `docs/.vitepress/{cache,dist}/`.

Deployment target stays provisional per ADR-0022; the workflow
publishes a `docs-site` artifact for now. The rsync step lands
once the future infrastructure ADR locks the host.

.gitignore

@@ -10,6 +10,10 @@ tmp/
 .nx/cache/
 .nx/workspace-data/
 
+# VitePress (docs site per ADR-0022)
+docs/.vitepress/cache/
+docs/.vitepress/dist/
+
 # Test artifacts
 coverage/
 .test-output/