[Issacboy123]

Summary

AgentSession.prompt() could not be cancelled reliably during asynchronous preflight, allowing overlapping work or a user/provider turn to begin after cancellation. This PR adds an optional prompt signal with synchronous lifecycle reservation, pre-start cancellation through authentication and compaction, and handoff to existing abort settlement after agent_start.

Reasoning

The prompt boundary is the narrowest place that owns admission, preflight, and active-run settlement. Reserving there closes overlap races without changing established queueing for unsignaled prompts. Pre-start cancellation preserves the caller's exact reason, while post-start cancellation reuses the session abort path so persistence and event semantics still have one terminal outcome. A rollback-capable compaction append is necessary because cancellation may arrive after persistence but before post-save extension notification completes.

Changes

• Added PromptOptions.signal and synchronous reservations covering prompt preflight through settlement.
• Rejected conflicting signaled prompts while preserving concurrent unsignaled preflight and existing active-run steering and follow-up behavior.
• Propagated cancellation through authentication and pre-prompt compaction, with one prompt-owned abort listener removed on every terminal path.
• Preserved exact pre-start abort reasons, pending next-turn messages, and the absence of user or provider turns after cancellation.
• Added transactional compaction append and rollback support so cancelled preflight leaves no completed compaction entry, including while compaction hooks are pending.
• Added focused prompt, callback-ordering, queueing, authentication, compaction, state-restoration, and listener-cleanup regressions.
• Documented the prompt signal contract in the SDK guide and recorded the fix in the changelog.

Testing

• node ../../node_modules/vitest/dist/cli.js --run test/suite/agent-session-prompt.test.ts test/session-manager/compaction-rollback.test.ts
• node ../../node_modules/vitest/dist/cli.js --run test/suite/agent-session-prompt.test.ts test/suite/subagent-services.test.ts test/suite/subagent-registration.test.ts test/suite/subagent-coordination.test.ts test/suite/subagent-parent-messaging.test.ts test/subagent-controller.test.ts
• npm run build
• npm run check