feat(evidence): add evidence infrastructure - types, append, query, retention

Phase 3 Task 3.1:
- Create src/evidence-log.ts with EvidenceEventType, EvidencePhase,
  EvidenceOutcome, MemoryEvidenceRef, EvidenceRelation, EvidenceEventV1,
  EvidenceEventInput types
- Add appendEvidenceEvent/appendEvidenceEvents with safe write, privacy
  hashing (SHA-256 truncated), textPreview redaction, bounded retention
- Add queryEvidenceEvents, summarizeMemoryEvidence, traceMemoryLifecycle
- Add workspaceEvidenceLogPath to src/paths.ts
- Add 8 evidence-log tests: round-trip, privacy, query, resilience, retention
- Relations limited to wiring roles only (no kind/derived_from/validates)
- 253 tests pass

Author: sdwolf4103