refactor(memory): promote on_session_reset to base provider hook

Replace hasattr-forked OpenViking-specific paths with a proper base-class
hook. Collapse the two agent wrappers into a single rotate_memory_session
so callers don't orchestrate commit + rebind themselves.

- MemoryProvider: add on_session_reset(new_session_id) as a default no-op
- MemoryManager: on_session_reset fans out unconditionally (no hasattr,
  no builtin skip — base no-op covers it)
- OpenViking: rename reset_session -> on_session_reset; drop the explicit
  POST /api/v1/sessions (OV auto-creates on first message) and the two
  debug raise_for_status wrappers
- AIAgent: collapse commit_memory_session + reinitialize_memory_session
  into rotate_memory_session(new_sid, messages)
- cli.py / run_agent.py: replace hasattr blocks and the split calls with
  a single unconditional rotate_memory_session call; compression path
  now passes the real messages list instead of []
- tests: align with on_session_reset, assert reset does NOT POST /sessions

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

agent/memory_provider.py

@@ -160,6 +160,15 @@ 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.