feat(06-01): add session management service with rotation, thresholds, and delta tracking
- Create createSessionManagementService factory with accumulateTokenUsage,
evaluateRotation (configurable thresholds per D-01), generateHandoff,
rotateSession, getHandoffContext, snapshotUsage, getUsageDelta
- Export DEFAULT_THRESHOLDS (maxRuns: 20, maxTokens: 500k, maxAgeHours: 72) and RotationThresholds type
- Adapter nativeContextManagement exemption skips rotation for Claude/Codex (D-04)
- Handoff markdown capped at 2000 chars with structured sections (D-06)
- Add 14 unit tests for service: rotation thresholds, adapter exemption, configurable
thresholds, token accumulation, handoff generation, session rotation, usage delta
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>