Persistence, memory management, guardrails, cost tracking, and an audit
event log for PhoenixAI conversations.

Key accomplishments:
- Adapter-based storage (ETS + Ecto) with 5 optional sub-behaviours
- Memory strategies with pipeline composition
- Long-term memory: facts and AI-generated user profiles
- Guardrails: token budget, cost budget, rate limiting
- Cost tracking with Decimal arithmetic
- Append-only event log with cursor pagination and PII redaction
- converse/3 single-function pipeline
- TelemetryHandler + HandlerGuardian
- Complete ExDoc documentation and GitHub Actions CI

48/48 requirements satisfied. 376 tests. 5,350 LOC.