fix(openviking): commit session on /new and context compression

The OpenViking memory provider extracts memories when its session is
committed (POST /api/v1/sessions/{id}/commit).  Before this fix, the
CLI had two code paths that changed the active session_id without ever
committing the outgoing OpenViking session:

1. /new (new_session() in cli.py) — called flush_memories() to write
   MEMORY.md, then immediately discarded the old session_id.  The
   accumulated OpenViking session was never committed, so all context
   from that session was lost before extraction could run.

2. /compress and auto-compress (_compress_context() in run_agent.py) —
   split the SQLite session (new session_id) but left the OpenViking
   provider pointing at the old session_id with no commit, meaning all
   messages synced to OpenViking were silently orphaned.

The gateway already handles session commit on /new and /reset via
shutdown_memory_provider() on the cached agent; the CLI path did not.

Fix: introduce a lightweight session-transition lifecycle alongside
the existing full shutdown path:

- OpenVikingMemoryProvider.reset_session(new_session_id): waits for
  in-flight background threads, resets per-session counters, and
  creates the new OV session via POST /api/v1/sessions — without
  tearing down the HTTP client (avoids connection overhead on /new).

- MemoryManager.restart_session(new_session_id): calls reset_session()
  on providers that implement it; falls back to initialize() for
  providers that do not.  Skips the builtin provider (no per-session
  state).

- AIAgent.commit_memory_session(messages): wraps
  memory_manager.on_session_end() without shutdown — commits OV session
  for extraction but leaves the provider alive for the next session.

- AIAgent.reinitialize_memory_session(new_session_id): wraps
  memory_manager.restart_session() — transitions all external providers
  to the new session after session_id has been assigned.

Call sites:
- cli.py new_session(): commit BEFORE session_id changes, reinitialize
  AFTER — ensuring OV extraction runs on the correct session and the
  new session is immediately ready for the next turn.
- run_agent._compress_context(): same pattern, inside the
  if self._session_db: block where the session_id split happens.

/compress and auto-compress are functionally identical at this layer:
both call _compress_context(), so both are fixed by the same change.

Tests added to tests/agent/test_memory_provider.py:
- TestMemoryManagerRestartSession: reset_session() routing, builtin
  skip, initialize() fallback, failure tolerance, empty-manager noop.
- TestOpenVikingResetSession: session_id update, per-session state
  clear, POST /api/v1/sessions call, API failure tolerance, no-client
  noop.

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

agent/memory_manager.py

@@ -281,6 +281,28 @@ class MemoryManager:
                     provider.name, e,
                 )
 
+    def restart_session(self, new_session_id: str) -> None:
+        """Transition external providers to a new session without full teardown.
+
+        Must be called AFTER on_session_end() has committed the old session.
+        Providers that implement reset_session() are transitioned cheaply
+        (HTTP client kept alive); others fall back to a full initialize().
+        The builtin provider is skipped — it has no per-session state.
+        """
+        for provider in self._providers:
+            if provider.name == "builtin":
+                continue
+            try:
+                if hasattr(provider, "reset_session"):
+                    provider.reset_session(new_session_id)
+                else:
+                    provider.initialize(session_id=new_session_id)
+            except Exception as e:
+                logger.debug(
+                    "Memory provider '%s' restart_session failed: %s",
+                    provider.name, e,
+                )
+
     def on_pre_compress(self, messages: List[Dict[str, Any]]) -> str:
         """Notify all providers before context compression.