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)

setup/platform.ts

@@ -100,7 +100,7 @@ export function getServiceManager(): ServiceManager {
 
 export function getNodePath(): string {
   try {
-    return execSync('command -v node', { encoding: 'utf-8' }).trim();
+    return execSync('command -v bun', { encoding: 'utf-8' }).trim();
   } catch {
     return process.execPath;
   }