feat: migrate runtime from Node.js to Bun

- Replace better-sqlite3 with bun:sqlite (native, no native addon build)
- Change all spawn('node') to spawn('bun') for agent processes
- Update package.json scripts: node→bun, tsx→bun, npm→bun
- Add bun-types for tsc compatibility
- Add bun:sqlite→better-sqlite3 shim for vitest (tests run on Node.js)
- Update Dockerfile: install bun alongside Node.js (CLIs need Node)
- Update setup/platform.ts: getNodePath() resolves bun binary
- Remove better-sqlite3 from production dependencies (devDep only for tests)

src/agent-runner.ts

@@ -178,7 +178,7 @@ export async function runAgentProcess(
   fs.mkdirSync(logsDir, { recursive: true });
 
   return new Promise((resolve) => {
-    const proc = spawn('node', [distEntry], {
+    const proc = spawn('bun', [distEntry], {
       stdio: ['pipe', 'pipe', 'pipe'],
       cwd: runnerDir,
       env,