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/__tests__/llm-parse.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, test } from "bun:test";
+
+import type { LlmToolCall } from "../src/llm/types.js";
+
+/** Mirror OpenAI response shape for parser coverage via chatCompletionWithTools integration later. */
+describe("LlmToolCall shape", () => {
+  test("tool call record fields", () => {
+    const call: LlmToolCall = {
+      id: "call_1",
+      name: "read_file",
+      arguments: '{"path":"README.md"}',
+    };
+    expect(call.name).toBe("read_file");
+    expect(JSON.parse(call.arguments)).toEqual({ path: "README.md" });
+  });
+});