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/container-runner.ts

@@ -518,7 +518,7 @@ export async function runReviewerContainer(args: {
       }
     }
   }
-  execArgs.push(containerName, 'node', '/app/dist/index.js');
+  execArgs.push(containerName, 'bun', '/app/dist/index.js');
 
   return new Promise<AgentOutput>((resolve) => {
     const proc = spawn(CONTAINER_RUNTIME_BIN, execArgs, {