Add external MCP server support (stdio JSON-RPC)

Lets the AI connect to any external stdio MCP server (e.g. safaridriver
--mcp) configured in Settings, with tools auto-discovered and prefixed
by server slug. Includes crash detection with backoff restart (5s/15s/30s)
and a Settings UI to add/enable/disable/remove servers.

Fixes the temp-dir allowlist in MCPService.isPathAllowed to also match
/tmp and /private/tmp (not just NSTemporaryDirectory(), which resolves
to a different per-user Darwin temp dir) so the MCP file tools can
actually read files external servers and image generation write there.
Also switches the Add Server sheet's argument parsing to a quote-aware
tokenizer so args containing spaces survive intact.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

oAI/oAIApp.swift

@@ -37,6 +37,9 @@ struct oAIApp: App {
         // Start email handler on app launch
         EmailHandlerService.shared.start()
 
+        // Start external MCP servers
+        Task { @MainActor in ExternalMCPManager.shared.startAll() }
+
         // Sync Git changes on app launch (pull + import)
         Task {
             await GitSyncService.shared.syncOnStartup()
@@ -56,7 +59,7 @@ struct oAIApp: App {
                 }
                 #if os(macOS)
                 .onReceive(NotificationCenter.default.publisher(for: NSApplication.willTerminateNotification)) { _ in
-                    // Trigger auto-save on app quit
+                    Task { @MainActor in ExternalMCPManager.shared.stopAll() }
                     Task {
                         await chatViewModel.onAppWillTerminate()
                     }