feat(protocol): AgentFn<Opt> type + createAgentAdapter bridge

Add AgentFn<Opt = void> as the formal agent boundary type:
- Input: ThreadContext (fixed), Output: string (fixed)
- Opt: agent-specific structured options (e.g. { workspace } for Cursor)

Add createAgentAdapter<Opt>(agent, extract) → AdapterFn bridge in
workflow-util-agent, plus createSimpleAgentAdapter for Opt = void.

Also fixes: workflow-cas composite flag + cursor tsconfig reference.

Refs #252

packages/workflow-agent-cursor/tsconfig.json

@@ -6,5 +6,9 @@
     "composite": true
   },
   "include": ["src/**/*.ts"],
-  "references": [{ "path": "../workflow-runtime" }, { "path": "../workflow-util-agent" }]
+  "references": [
+    { "path": "../workflow-cas" },
+    { "path": "../workflow-runtime" },
+    { "path": "../workflow-util-agent" }
+  ]
 }