fix(session): fsync sessions on graceful shutdown to prevent data loss

On filesystems with write-back caching (rclone VFS, NFS, FUSE mounts)
the OS page cache may buffer recent session writes. If the process is
killed before the cache flushes, the most recent conversation turns are
silently lost — causing the agent to "forget" recent context and
respond to stale history on the next startup.

Changes:

- session/manager.py: add fsync=True option to save() that flushes the
  file and its parent directory to durable storage. Add flush_all() that
  re-saves every cached session with fsync. Default save() behavior is
  unchanged (no fsync) to avoid performance regression in normal
  operation.

- cli/commands.py: call agent.sessions.flush_all() in the gateway
  shutdown finally block, after stopping heartbeat/cron/channels.

- tests/session/test_session_fsync.py: 8 tests covering fsync flag
  behavior, flush_all with empty/multiple/errored sessions, and
  data survival across simulated process restart.

- tests/cli/test_commands.py: add sessions attribute to _FakeAgentLoop
  so the gateway health endpoint test passes with the new shutdown
  flush.

tests/cli/test_commands.py

@@ -1288,10 +1288,15 @@ def test_gateway_health_endpoint_binds_and_serves_expected_responses(
         async def run(self) -> None:
             return None
 
+    class _FakeSessionManager:
+        def flush_all(self) -> int:
+            return 0
+
     class _FakeAgentLoop:
         def __init__(self, **_kwargs) -> None:
             self.model = "test-model"
             self.dream = _FakeDream()
+            self.sessions = _FakeSessionManager()
 
         async def run(self) -> None:
             await asyncio.Event().wait()