feat(hook): Add contract for lifecycle events

[prathshenoy]

Summary

What:

• Define the shared event format every domain publishes for fire-and-forget lifecycle notifications, carrying origin, type, timing, a staleness marker, and open per-event detail.
• Derive an event's identity from the transition it describes, so replaying a transition produces the same identity and duplicate deliveries collapse.

Why:

• Enable integrations to react to pipeline milestones without adding call sites that can stall or fail a merge.
• Establish one schema and one identity rule before any producer ships, so a consumer spanning several domains needs only one reader.

Test Plan

• Add unit tests.

Revert Plan

• Revert this PR. No topic is registered and no message of this shape exists on any queue.

Issues

• CODEM-416

Stack

1. @ feat(hook): Add contract for lifecycle events #606
2. feat(hook): Deliver events to integrations #607
3. feat(orchestrator): Consume hook events #608