feat(memory): add Agent Reflection Engine for cross-session learning

Story 9.6 of Epic 9 (Persistent Memory Layer).
Phase 2 of Agent Immortality Protocol (#482).

The Reflection Engine enables agents to learn from past executions:
- Record execution reflections with outcome, strategy, duration
- Extract recurring patterns (success/failure clusters)
- Recommend strategies before similar tasks based on history
- Track performance trends per agent and task type
- Inject relevant reflections as context before execution
- Auto-prune stale reflections beyond retention window

44 unit tests covering all features including edge cases.

Author: nikolasdehor