feat(v2): migrate container runtime to Bun, improve image build surface

Container side:
- agent-runner switches to Bun. Drops better-sqlite3 (native compile gone),
  drops tsc build step in-image AND the tsc-on-every-session-wake in the
  entrypoint — bun runs src/index.ts directly. bun:sqlite replaces
  better-sqlite3; cross-mount DB invariants (journal_mode=DELETE, busy_timeout)
  preserved. Named params converted from @name to $name because bun:sqlite
  does not auto-strip the prefix the way better-sqlite3 does.
- Tests ported from vitest to bun:test (only describe/it/expect/before/afterEach
  used, API-compatible). vitest.config.ts excludes container/agent-runner/.
- bun.lock replaces pnpm-lock.yaml + pnpm-workspace.yaml under
  container/agent-runner/. Host pnpm workspace does NOT include this tree.

Dockerfile improvements (independent of Bun but bundled while touching the file):
- tini as PID 1 for correct SIGTERM propagation (prevents half-written
  outbound.db on shutdown).
- Extracted entrypoint.sh — readable and diffable vs the old inline printf.
- BuildKit cache mounts for apt + bun install + pnpm install.
- --no-install-recommends on apt, pinned CLAUDE_CODE_VERSION, AGENT_BROWSER,
  VERCEL, BUN_VERSION.
- CJK fonts (~200MB) behind ARG INSTALL_CJK_FONTS=false; build.sh reads from
  .env; setup/container.ts reads the same .env so /setup and manual rebuild
  stay in sync.
- PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD=1 in case any postinstall tries to pull a
  redundant Chromium.
- /home/node 755 (was 777).

Host side:
- src/container-runner.ts dynamic spawn command collapses from
  `pnpm exec tsc --outDir /tmp/dist … && node /tmp/dist/index.js` to
  `exec bun run /app/src/index.ts` — cold start ~200-500ms faster per wake.

CI:
- oven-sh/setup-bun@v2 alongside Node/pnpm. Adds explicit container
  typecheck (was documented in CLAUDE.md, not enforced) and `bun test` for
  agent-runner tests.

container/agent-runner/src/poll-loop.test.ts

@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { describe, it, expect, beforeEach, afterEach } from 'bun:test';
 
 import { initTestSessionDb, closeSessionDb, getInboundDb, getOutboundDb } from './db/connection.js';
 import { getPendingMessages, markCompleted } from './db/messages-in.js';