refactor(memory): drop on_session_reset — commit-only is enough

OV transparently handles message history across /new and /compress: old
messages stay in the same session and extraction is idempotent, so there's
no need to rebind providers to a new session_id. The only thing the
session boundary actually needs is to trigger extraction.

- MemoryProvider / MemoryManager: remove on_session_reset hook
- OpenViking: remove on_session_reset override (nothing to do)
- AIAgent: replace rotate_memory_session with commit_memory_session
  (just calls on_session_end, no rebind)
- cli.py / run_agent.py: single commit_memory_session call at the
  session boundary before session_id rotates
- tests: replace on_session_reset coverage with routing tests for
  MemoryManager.on_session_end

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

plugins/memory/openviking/__init__.py

@@ -516,22 +516,6 @@ class OpenVikingMemoryProvider(MemoryProvider):
         except Exception as e:
             return tool_error(str(e))
 
-    def on_session_reset(self, new_session_id: str) -> None:
-        """Rebind per-session state to new_session_id. OV auto-creates the
-        session when the first message is added, so no create call here."""
-        for t in (self._sync_thread, self._prefetch_thread):
-            if t and t.is_alive():
-                t.join(timeout=5.0)
-
-        self._session_id = new_session_id
-        self._turn_count = 0
-        self._prefetch_result = ""
-        self._sync_thread = None
-        self._prefetch_thread = None
-
-        global _last_active_provider
-        _last_active_provider = self
-
     def shutdown(self) -> None:
         # Wait for background threads to finish
         for t in (self._sync_thread, self._prefetch_thread):