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)

vitest.config.ts

@@ -1,6 +1,12 @@
+import path from 'path';
 import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
+  resolve: {
+    alias: {
+      'bun:sqlite': path.resolve(__dirname, 'test/bun-sqlite-shim.ts'),
+    },
+  },
   test: {
     include: [
       'src/**/*.test.ts',