Extract two self-contained subsystems from agent-session.ts (3,367 -> 2,737 lines):
- RetryHandler: auto-retry with exponential backoff, credential rotation,
and cross-provider fallback logic
- CompactionOrchestrator: manual/auto compaction, overflow recovery, and
extension integration for custom compaction providers
Also add shared getErrorMessage() utility to replace repeated
`err instanceof Error ? err.message : String(err)` patterns.
The extracted modules receive AgentSession state via dependency injection
interfaces, avoiding state duplication. AgentSession remains the coordinator
that delegates to these modules.
