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/environment.test.ts

@@ -1,7 +1,7 @@
 import { describe, it, expect, beforeEach } from 'vitest';
 import fs from 'fs';
 
-import Database from 'better-sqlite3';
+import { Database } from 'bun:sqlite';
 
 /**
  * Tests for the environment check step.
@@ -18,7 +18,7 @@ describe('environment detection', () => {
 });
 
 describe('registered groups DB query', () => {
-  let db: Database.Database;
+  let db: Database;
 
   beforeEach(() => {
     db = new Database(':memory:');