docs(v2): runtime-split guide, CLAUDE.md gotchas, setup CJK autodetect

- docs/v2-build-and-runtime.md: new — runtime split rationale (Node host,
  Bun container), lockfile topology, supply-chain trade-offs, image build
  surface, two session-wake paths, CI shape, key invariants. Indexed from
  CLAUDE.md v2 Docs Index.
- CLAUDE.md: Container Runtime (Bun) section with trigger/action gotchas
  a contributor editing the container must know (named-param prefix rule,
  bun:test vs vitest, bun.lock regeneration, no minimumReleaseAge for the
  Bun tree, no tsc build step, DELETE pragma invariant). CJK font support
  section for Claude sessions outside of /setup to proactively offer when
  they detect CJK signals. Development section updated with Bun commands.
- .claude/skills/setup/SKILL.md: step 3b — auto-enable CJK fonts without
  asking if the user is already writing in CJK; otherwise ask only on clear
  signals (CJK timezone from step 2a). 3c renumbered from old 3b.

.claude/skills/setup/SKILL.md

@@ -89,7 +89,23 @@ Run `pnpm exec tsx setup/index.ts --step timezone` and parse the status block.
   - macOS: install via `brew install --cask docker`, then `open -a Docker` and wait for it to start. If brew not available, direct to Docker Desktop download at https://docker.com/products/docker-desktop
   - Linux: install with `curl -fsSL https://get.docker.com | sh && sudo usermod -aG docker $USER`. Note: user may need to log out/in for group membership.
 
-### 3b. Build and test
+### 3b. CJK fonts
+
+Agent containers skip CJK fonts by default (~200MB saved). Without them, Chromium-rendered screenshots and PDFs show tofu for Chinese/Japanese/Korean.
+
+- **User writing to you in Chinese, Japanese, or Korean** → enable without asking. Mention it briefly.
+- **Resolved timezone from step 2a is a CJK region** (`Asia/Tokyo`, `Asia/Shanghai`, `Asia/Hong_Kong`, `Asia/Taipei`, `Asia/Seoul`) or other signal short of active CJK use → ask: "Enable CJK fonts? Adds ~200MB, lets the agent render CJK in screenshots and PDFs."
+- **Otherwise** → skip.
+
+To enable, write `INSTALL_CJK_FONTS=true` to `.env`:
+
+```bash
+grep -q '^INSTALL_CJK_FONTS=' .env && sed -i.bak 's/^INSTALL_CJK_FONTS=.*/INSTALL_CJK_FONTS=true/' .env && rm -f .env.bak || echo 'INSTALL_CJK_FONTS=true' >> .env
+```
+
+The next step's build picks it up automatically.
+
+### 3c. Build and test
 
 Run `pnpm exec tsx setup/index.ts --step container -- --runtime docker` and parse the status block.