Add idempotent retrospectives for terminal workflows

[eshulman2]

Summary

• add opt-in retrospective reporting at the worker's terminal-outcome boundary
• centrally distinguish successful/blocked terminal outcomes from human approval and review waits
• use typed, bounded, redacted evidence and an initial deterministic zero-model-cost analyzer
• use generation-aware idempotency keys so duplicate events do not duplicate reports while a later retry can produce a new outcome generation
• keep retrospective failures non-blocking and improvement-task creation separately disabled by default

Forge architecture

Retrospectives run outside individual LangGraph graphs after the worker observes a terminal outcome. They do not change workflow routing or completion state.

Test plan

• 25 retrospective and worker tests pass
• waiting-state and retry-generation behavior covered
• Ruff passes on changed implementation and tests

Closes #261