fix(clients): write Antigravity config to ~/.gemini/config/ (post-2.x migration)

Antigravity 2.x moved its MCP config from ~/.gemini/antigravity/mcp_config.json
to a dedicated ~/.gemini/config/mcp_config.json. The migration drops a
`.migrated` marker in the new location and renames the prior folder to
`antigravity-backup`; the old path keeps Antigravity's runtime state
(conversations/, scratch/, brain/, etc.) but is no longer read for MCP.
Confirmed empirically on a real machine: ~/.gemini/config/mcp_config.json
shows `mcpServers: {}` (Antigravity reads from here) while our writes have
been landing in the legacy ~/.gemini/antigravity/mcp_config.json that
Antigravity now ignores. Community report matches this exactly: "in
.gemini there are two folders, antigravity and config; antigravity is not
where mcp.json is supposed to be, there is another config folder."

Also override IsInstalled. The inherited ParentDirectoryExists check
points at ~/.gemini/config/, which is only created on Antigravity's first
launch — on a fresh machine that has the app installed but never opened,
detection false-negatives and ConfigureAllDetectedClients skips
Antigravity entirely. Check ~/.antigravity/ (installer-created), the new
config dir, the legacy config dir, or /Applications/Antigravity.app on
macOS; any one of those is conclusive.

Author: Scriptwonder

MCPForUnity/Editor/Clients/Configurators/AntigravityConfigurator.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Runtime.InteropServices;
 using MCPForUnity.Editor.Constants;
 using MCPForUnity.Editor.Models;
 using UnityEditor;
@@ -9,18 +10,44 @@ namespace MCPForUnity.Editor.Clients.Configurators
 {
     public class AntigravityConfigurator : JsonFileMcpConfigurator
     {
+        // Antigravity 2.x migrated its MCP config from ~/.gemini/antigravity/mcp_config.json
+        // (where Antigravity also stores its own runtime state — conversations, scratch, etc.)
+        // to a dedicated ~/.gemini/config/mcp_config.json. The migration drops a `.migrated`
+        // marker in the new location and renames the previous folder to `antigravity-backup`.
+        // The old path is no longer read by Antigravity at all, so writing there silently
+        // fails to register UnityMCP on every modern install.
         public AntigravityConfigurator() : base(new McpClient
         {
             name = "Antigravity",
-            windowsConfigPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile), ".gemini", "antigravity", "mcp_config.json"),
-            macConfigPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile), ".gemini", "antigravity", "mcp_config.json"),
-            linuxConfigPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile), ".gemini", "antigravity", "mcp_config.json"),
+            windowsConfigPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile), ".gemini", "config", "mcp_config.json"),
+            macConfigPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile), ".gemini", "config", "mcp_config.json"),
+            linuxConfigPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile), ".gemini", "config", "mcp_config.json"),
             HttpUrlProperty = "serverUrl",
             DefaultUnityFields = { { "disabled", false } },
             StripEnvWhenNotRequired = true
         })
         { }
 
+        // Detect Antigravity itself, not just its config dir. ~/.gemini/config/ is created on
+        // first launch of Antigravity 2.x, so the inherited ParentDirectoryExists check
+        // false-negatives on a fresh install where the user hasn't opened Antigravity yet.
+        // ~/.antigravity/ is created by the installer (VS-Code-style support dir) and the
+        // macOS app bundle is dropped by the installer; either is conclusive evidence that
+        // Antigravity is installed, regardless of whether it has been launched.
+        public override bool IsInstalled
+        {
+            get
+            {
+                string home = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
+                if (Directory.Exists(Path.Combine(home, ".antigravity"))) return true;
+                if (Directory.Exists(Path.Combine(home, ".gemini", "config"))) return true;
+                if (Directory.Exists(Path.Combine(home, ".gemini", "antigravity"))) return true;
+                if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+                    return Directory.Exists("/Applications/Antigravity.app");
+                return false;
+            }
+        }
+
         public override IList<string> GetInstallationSteps() => new List<string>
         {
             "Open Antigravity",