feat: add @uncaged/workflow-agent-builtin package

Built-in role agent that uses workflow config models directly,
with its own tool-calling run loop. No external agent dependency.

- OpenAI-compatible chat completion client with tool_calls support
- P0 toolkit: read_file, write_file, run_command
- Integrates via createAgent factory from workflow-agent-kit
- CAS detail recording for each turn
- Path sandboxing and shell opt-in (UWF_BUILTIN_ALLOW_SHELL)

packages/workflow-agent-builtin/package.json

@@ -0,0 +1,34 @@
+{
+  "name": "@uncaged/workflow-agent-builtin",
+  "version": "0.5.0",
+  "files": [
+    "src",
+    "dist",
+    "package.json"
+  ],
+  "type": "module",
+  "bin": {
+    "uwf-builtin": "./src/cli.ts"
+  },
+  "exports": {
+    ".": {
+      "bun": "./src/index.ts",
+      "types": "./dist/index.d.ts",
+      "import": "./dist/index.js"
+    }
+  },
+  "scripts": {
+    "test": "bun test"
+  },
+  "dependencies": {
+    "@uncaged/json-cas": "^0.4.0",
+    "@uncaged/workflow-agent-kit": "workspace:^",
+    "@uncaged/workflow-util": "workspace:^"
+  },
+  "devDependencies": {
+    "typescript": "^5.8.3"
+  },
+  "publishConfig": {
+    "access": "public"
+  }
+}