fix: agent bin fields point to dist/cli.js instead of src/cli.ts

All three agent packages had bin pointing to ./src/cli.ts (bun-era
leftover). Node cannot execute .ts files directly, causing
ERR_MODULE_NOT_FOUND when spawning agents.

Closes #78

packages/agent-builtin/package.json

@@ -8,7 +8,7 @@
   ],
   "type": "module",
   "bin": {
-    "uwf-builtin": "./src/cli.ts"
+    "uwf-builtin": "./dist/cli.js"
   },
   "exports": {
     ".": {