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/register.ts

@@ -7,7 +7,7 @@
 import fs from 'fs';
 import path from 'path';
 
-import Database from 'better-sqlite3';
+import { Database } from 'bun:sqlite';
 
 import { STORE_DIR } from '../src/config.js';
 import { isValidGroupFolder } from '../src/group-folder.js';