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)

tsconfig.json

@@ -23,6 +23,7 @@
     { "path": "packages/workflow-moderator" },
     { "path": "packages/workflow-agent-kit" },
     { "path": "packages/workflow-agent-hermes" },
+    { "path": "packages/workflow-agent-builtin" },
     { "path": "packages/cli-workflow" }
   ]
 }