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/ViewModels/ChatViewModel.swift

@@ -801,6 +801,7 @@ Don't narrate future actions ("Let me...") - just use the tools.
         let bashActive = settings.bashEnabled
         let personalDataActive = settings.calendarEnabled || settings.remindersEnabled || settings.contactsEnabled || settings.locationMapsEnabled
         let researchAgentsActive = settings.agentsEnabled
+        let externalMCPActive = !settings.externalMCPServers.filter { $0.isEnabled }.isEmpty
         // Dedicated images API path (OpenRouter /images endpoint — separate from chat completions)
         if selectedModel?.capabilities.usesImagesAPI == true,
            let orProvider = provider as? OpenRouterProvider {
@@ -809,7 +810,7 @@ Don't narrate future actions ("Let me...") - just use the tools.
         }
 
         let modelSupportTools = selectedModel?.capabilities.tools ?? false
-        if modelSupportTools && (anytypeActive || bashActive || personalDataActive || researchAgentsActive || (mcpActive && !mcp.allowedFolders.isEmpty)) {
+        if modelSupportTools && (anytypeActive || bashActive || personalDataActive || researchAgentsActive || externalMCPActive || (mcpActive && !mcp.allowedFolders.isEmpty)) {
             generateAIResponseWithTools(provider: provider, modelId: modelId)
             return
         }
@@ -1360,6 +1361,12 @@ Don't narrate future actions ("Let me...") - just use the tools.
                     systemParts.append("You have access to the user's Anytype knowledge base through tool calls (anytype_* tools). You can search across all spaces, list spaces, get objects, and create or update notes, tasks, and pages. Use these tools proactively when the user asks about their notes, tasks, or knowledge base.")
                 }
 
+                let activeExternalServers = settings.externalMCPServers.filter { $0.isEnabled }
+                if !activeExternalServers.isEmpty {
+                    let names = activeExternalServers.map { $0.name }.joined(separator: ", ")
+                    systemParts.append("You have access to external tools from MCP servers: \(names). Their tools are prefixed with the server slug (e.g. safari_navigate_to_url). Use them proactively when the user's request relates to what those servers provide.")
+                }
+
                 var systemContent = systemParts.joined(separator: "\n\n")
 
                 // Append the complete system prompt (default + custom)