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-protocol/src/index.ts

@@ -13,6 +13,7 @@ export type {
   AdapterFn,
   AdvanceOutcome,
   AgentContext,
+  AgentFn,
   CasStore,
   ExtractFn,
   ExtractResult,

packages/workflow-protocol/src/types.ts

@@ -151,6 +151,15 @@ export type RoleFn<T> = (ctx: ThreadContext, runtime: WorkflowRuntime) => Promis
 
 export type AdapterFn = <T>(prompt: string, schema: z.ZodType<T>) => RoleFn<T>;
 
+/**
+ * Core agent function. Input is always {@link ThreadContext}, output is always string.
+ * `Opt` captures agent-specific structured options.
+ * Agents with no extra options use `AgentFn` (Opt defaults to void).
+ */
+export type AgentFn<Opt = void> = Opt extends void
+  ? (ctx: ThreadContext) => Promise<string>
+  : (ctx: ThreadContext, options: Opt) => Promise<string>;
+
 export type AdapterBinding = {
   adapter: AdapterFn;
   overrides: Partial<Record<string, AdapterFn>> | null;