feat(util-agent): extend AgentOptions with fork / cleanup (Phase 2a)

Add AgentForkFn and AgentCleanupFn type aliases. Extend AgentOptions
with fork: AgentForkFn | null and cleanup: AgentCleanupFn | null
fields. Add getAskSessionId / setAskSessionId session-cache helpers
using <stepHash>:ask key shape (coexists with exec sessions in the
same per-agent cache file). All four adapters pass fork: null,
cleanup: null — real wiring lands in Phase 2b. Resolves #145.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/agent-mock/src/mock-agent.ts

@@ -125,5 +125,7 @@ export function createMockAgent(mockDataPath: string): () => Promise<void> {
     name: "mock",
     run,
     continue: continueRun,
+    fork: null,
+    cleanup: null,
   });
 }