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>

app/services/streaming.py

@@ -142,6 +142,24 @@ class StreamHandler:
             )
         return result
 
+    def get_partial_message(self) -> Message | None:
+        """Return whatever content/tool_calls have been accumulated so far.
+
+        Useful for mid-stream interruption recovery — returns None if nothing
+        has been accumulated yet.
+
+        Returns:
+            Partial assistant Message, or None if no content accumulated.
+        """
+        tool_calls = self._build_tool_calls() or None
+        if not self._accumulated_content and not tool_calls:
+            return None
+        return Message(
+            role="assistant",
+            content=self._accumulated_content or None,
+            tool_calls=tool_calls,
+        )
+
     @property
     def usage(self) -> TokenUsage | None:
         """Token usage reported by the API, if available."""