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>
