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>

agent/memory_manager.py

@@ -281,22 +281,6 @@ class MemoryManager:
                     provider.name, e,
                 )
 
-    def on_session_reset(self, new_session_id: str) -> None:
-        """Notify all providers of a session reset.
-
-        Called after on_session_end() has committed the previous session.
-        Providers with per-session state override on_session_reset to rebind
-        it cheaply (default is a no-op on the base class).
-        """
-        for provider in self._providers:
-            try:
-                provider.on_session_reset(new_session_id)
-            except Exception as e:
-                logger.debug(
-                    "Memory provider '%s' on_session_reset failed: %s",
-                    provider.name, e,
-                )
-
     def on_pre_compress(self, messages: List[Dict[str, Any]]) -> str:
         """Notify all providers before context compression.
 

agent/memory_provider.py

@@ -160,15 +160,6 @@ class MemoryProvider(ABC):
         (CLI exit, /reset, gateway session expiry).
         """
 
-    def on_session_reset(self, new_session_id: str) -> None:
-        """Transition to a new session without full teardown.
-
-        Called after on_session_end() has committed the previous session
-        (e.g. /new, context compression). Providers with per-session state
-        override to rebind counters/IDs while keeping HTTP clients alive.
-        Default: no-op.
-        """
-
     def on_pre_compress(self, messages: List[Dict[str, Any]]) -> str:
         """Called before context compression discards old messages.