Add Phase 7: polish and hardening — retry, truncation, sessions, shutdown

- Config extensions: retry backoff, truncation threshold, session persistence
- LLM retry with exponential backoff + jitter on transient errors (5xx, connection)
- Conversation truncation: drops oldest messages preserving first user + recent N
- Session persistence: auto-save/restore with atomic writes, cleanup of old files
- Graceful shutdown: SIGTERM handler, cancel() on AgentLoop, save-on-exit
- Partial message recovery on mid-stream interruption
- New slash commands: /save, /session
- 18 new tests (5 retry, 5 truncation, 4 session, 4 integration workflows)
- README.md and docs/tools.md documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/test_agent_loop.py

@@ -62,6 +62,7 @@ def handler() -> MagicMock:
     mock.usage = None
     mock.had_reasoning_only = False
     mock.reset = MagicMock()
+    mock.get_partial_message = MagicMock(return_value=None)
     return mock