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/components/chat/ChatInput.tsx

@@ -1520,7 +1520,9 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({ onOpenSettings, scrollTo
             else if (commandName === 'compact' && currentSessionId) {
                 try {
                     if (!useConfigStore.getState().isConnected) {
-                        throw new Error("Connection lost. Please wait for reconnection.");
+                        const reason = useConfigStore.getState().lastDisconnectReason;
+                        const suffix = reason ? ` (${reason})` : " (never connected)";
+                        throw new Error(`Connection lost${suffix}. Please wait for reconnection.`);
                     }
                     const { opencodeClient } = await import('@/lib/opencode/client');
                     const sdk = opencodeClient.getSdkClient();