fix: enable psutil fallback for memory monitoring when macmon is missing on macOS

On Darwin, the psutil memory poller was disabled (memory_poll_rate=None),
relying entirely on macmon. When macmon is not installed, no memory data
was reported, causing nodes to show zero memory in the cluster state and
blocking shard placement.

Now falls back to psutil-based memory polling when macmon is not found.

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

src/exo/utils/info_gatherer/info_gatherer.py

@@ -388,6 +388,10 @@ class InfoGatherer:
             if IS_DARWIN:
                 if (macmon_path := shutil.which("macmon")) is not None:
                     tg.start_soon(self._monitor_macmon, macmon_path)
+                else:
+                    # macmon not installed — fall back to psutil for memory
+                    logger.warning("macmon not found, falling back to psutil for memory monitoring")
+                    self.memory_poll_rate = 1
                 tg.start_soon(self._monitor_system_profiler_thunderbolt_data)
                 tg.start_soon(self._monitor_thunderbolt_bridge_status)
                 tg.start_soon(self._monitor_rdma_ctl_status)