fix: surface disconnect reason, switch health probe to /global/health (#978)

- event-pipeline: onDisconnect now carries a reason tag (ws_closed:code=N, ws_error_frame, ws_closed_before_ready, sse_error)
- useConfigStore: store lastDisconnectReason, clear on reconnect
- send guards: embed reason in Connection lost toast so we can tell which path tripped
- lifecycle: isOpenCodeProcessHealthy hits /global/health (healthy flag) with 5s timeout instead of /session with 2s, avoids false restarts under stream load

packages/ui/src/sync/sync-context.tsx

@@ -1347,7 +1347,7 @@ export function SyncProvider(props: {
         handleEvent(directory, payload, childStores, routingIndex)
       },
       onReconnect: () => {
-        useConfigStore.setState({ isConnected: true })
+        useConfigStore.setState({ isConnected: true, lastDisconnectReason: null })
         for (const [dir, store] of childStores.children) {
           if (reconnectResyncing.has(dir)) continue
           if (getReconnectCandidateSessionIds(store.getState()).length === 0) continue
@@ -1362,8 +1362,8 @@ export function SyncProvider(props: {
             })
         }
       },
-      onDisconnect: () => {
-        useConfigStore.setState({ isConnected: false })
+      onDisconnect: (reason) => {
+        useConfigStore.setState({ isConnected: false, lastDisconnectReason: reason })
       },
     })
     return cleanup