feat(worker): add notify-termination and cancellation-token utilities

[WinterYukky]

Summary

Add two complementary session lifecycle primitives for cooperative shutdown and cancellation signaling.

Motivation

Long-running agent sessions need reliable mechanisms to:

1. Signal cancellation to in-flight operations (subprocess teardown, streaming abort) without waiting for the next polling interval
2. Perform graceful shutdown when the orchestrator requests termination — draining in-flight work before process exit with a configurable timeout and force-kill fallback

These primitives enable cleaner session lifecycle management and reduce orphaned subprocess accumulation.

Changes

• CancellationToken: cooperative cancellation signal with synchronous listener support. Listeners fire immediately on cancel(), enabling instant subprocess teardown without polling.
• notifyTermination: graceful shutdown handler that registers SIGTERM/SIGINT handlers, drains registered cleanup functions with a configurable timeout, and force-exits if cleanup exceeds the deadline.

Files changed (4 files, +449):

• packages/worker/src/common/cancellation-token.ts
• packages/worker/src/common/cancellation-token.test.ts
• packages/worker/src/common/notify-termination.ts
• packages/worker/src/common/notify-termination.test.ts

Testing

• 21 unit tests (8 cancellation-token + 13 notify-termination) — all passing
• TypeScript compilation passes
• Prettier format check passes

Notes

Self-contained. No dependencies on other PRs.

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