feat(web): render docs from repo-root docs/ as MDX SoT at build time

Replace the bespoke 16-section /docs page that pulled prose from
`messages/{locale}.json` with a build-time MDX renderer that reads the
canonical markdown in repo-root `docs/`. Each markdown file becomes
one section of the docs page, scrolled-to via the existing DocsShell
sidebar. Section data structure stays in `lib/docs-sections.ts` so the
sidebar / scroll-spy keeps working with no client changes.

Why this layout:

- One source of truth: `docs/guide/*.md`, `docs/reference/*.md`,
  `docs/manifesto.md`. Edits land in one place; the website redeploys
  pick them up automatically via the existing web-deploy workflow.
- Build-time only: `MDXRemote` is rendered inside an RSC and the page
  is statically generated (`●` SSG). Cloudflare Workers serves the
  rendered HTML; no MDX compiler runs at request time.
- next-intl unchanged for everything else: only the docs prose moves
  out. `mobileHeader` and `searchPlaceholder` strings stay in
  `messages/{locale}.json`; the 18 stale section keys are removed.

Files:

- web/lib/docs-sections.ts: 9 sections matching docs/ files, typed
  `DocSection` with `{ id, title, file }`.
- web/lib/docs-source.ts: `loadDocSource(file)` reads
  `<repo-root>/docs/<file>` at build time via `node:fs/promises`.
- web/components/docs/mdx-components.tsx: shadcn-styled overrides for
  every markdown element (h1-h4, p, a, ul/ol/li, blockquote, code, pre,
  table, hr, strong) so the rendered output matches the rest of the
  site.
- web/app/[locale]/docs/page.tsx: rewritten as an async RSC that loads
  every section's source in parallel and renders one MDXRemote per
  section inside DocsShell.
- web/messages/{en,ja,ko,zh}.json: `docs` key trimmed from 20 entries
  to 2 (mobileHeader, searchPlaceholder).
- web/package.json: + next-mdx-remote, + gray-matter.

Local verification: `bun run format:check`, `bun run lint`,
`bun run type-check`, `bun run build`, `bunx opennextjs-cloudflare
build` all pass; `/[locale]/docs` builds as static for all 4 locales
at 4.12 kB / 132 kB First Load.

web/lib/docs-sections.ts

@@ -1,39 +1,25 @@
-export const DOC_SECTION_IDS = [
-  "overview",
-  "quick-start",
-  "config-locations",
-  "agents",
-  "categories",
-  "skills",
-  "background-tasks",
-  "hooks",
-  "mcps",
-  "browser-automation",
-  "tmux",
-  "git-master",
-  "comment-checker",
-  "experimental",
-  "lsp",
-  "env-vars",
+export type DocSection = {
+  id: string
+  title: string
+  file: string
+}
+
+export const DOC_SECTIONS: readonly DocSection[] = [
+  { id: "overview", file: "guide/overview.md", title: "Overview" },
+  { id: "installation", file: "guide/installation.md", title: "Installation" },
+  { id: "orchestration", file: "guide/orchestration.md", title: "Orchestration" },
+  {
+    id: "agent-model-matching",
+    file: "guide/agent-model-matching.md",
+    title: "Agent / Model Matching",
+  },
+  { id: "team-mode", file: "guide/team-mode.md", title: "Team Mode" },
+  { id: "cli", file: "reference/cli.md", title: "CLI Reference" },
+  { id: "configuration", file: "reference/configuration.md", title: "Configuration" },
+  { id: "features", file: "reference/features.md", title: "Features" },
+  { id: "manifesto", file: "manifesto.md", title: "Manifesto" },
 ] as const
 
-export type DocSectionId = (typeof DOC_SECTION_IDS)[number]
+export const DOC_SECTION_IDS = DOC_SECTIONS.map((s) => s.id)
 
-export const DOC_SECTION_TITLE_KEYS: Record<DocSectionId, string> = {
-  overview: "overview",
-  "quick-start": "quickStart",
-  "config-locations": "configLocations",
-  agents: "agents",
-  categories: "categories",
-  skills: "skills",
-  "background-tasks": "backgroundTasks",
-  hooks: "hooks",
-  mcps: "mcps",
-  "browser-automation": "browserAutomation",
-  tmux: "tmux",
-  "git-master": "gitMaster",
-  "comment-checker": "commentChecker",
-  experimental: "experimental",
-  lsp: "lsp",
-  "env-vars": "envVars",
-}
+export type DocSectionId = (typeof DOC_SECTIONS)[number]["id"]