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)

runners/agent-runner/package.json

@@ -6,7 +6,7 @@
   "main": "dist/index.js",
   "scripts": {
     "build": "tsc",
-    "start": "node dist/index.js"
+    "start": "bun dist/index.js"
   },
   "dependencies": {
     "@anthropic-ai/claude-agent-sdk": "^0.2.81",

runners/codex-runner/package.json

@@ -6,7 +6,7 @@
   "main": "dist/index.js",
   "scripts": {
     "build": "tsc",
-    "start": "node dist/index.js"
+    "start": "bun dist/index.js"
   },
   "dependencies": {
     "@openai/codex": "^0.115.0"