Regenerate app icons from Icon Composer (#1005)

* Add Apple Icon Composer source file

Store the .icon project file in design/cmux.icon so the icon can be
edited in Icon Composer and regenerated from source.

* Regenerate all app icons from Icon Composer

Use ictool to render light/dark icon PNGs from the .icon source file
with proper macOS padding. Updates AppIcon, AppIcon-Debug (with DEV
banner), AppIconLight, and AppIconDark imagesets. Adds AppIcon.icon
to the Xcode project as a resource.

* Address review: fix trailing newline, remove .icon from bundle

- Add trailing newline to icon.json files
- Remove AppIcon.icon from Copy Bundle Resources (design-time only)

AppIcon.icon/icon.json

@@ -0,0 +1,35 @@
+{
+  "fill" : "automatic",
+  "groups" : [
+    {
+      "layers" : [
+        {
+          "glass" : false,
+          "image-name" : "cmux-icon-chevron 2.png",
+          "name" : "cmux-icon-chevron 2",
+          "position" : {
+            "scale" : 1,
+            "translation-in-points" : [
+              37.357790031201375,
+              -0.5
+            ]
+          }
+        }
+      ],
+      "shadow" : {
+        "kind" : "neutral",
+        "opacity" : 0.5
+      },
+      "translucency" : {
+        "enabled" : true,
+        "value" : 0.5
+      }
+    }
+  ],
+  "supported-platforms" : {
+    "circles" : [
+      "watchOS"
+    ],
+    "squares" : "shared"
+  }
+}

GhosttyTabs.xcodeproj/project.pbxproj

@@ -207,6 +207,7 @@
 		B8F266256A1A3D9A45BD840F /* SidebarHelpMenuUITests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SidebarHelpMenuUITests.swift; sourceTree = "<group>"; };
 		C0B4D9B1A1B2C3D4E5F60718 /* UpdatePillUITests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = UpdatePillUITests.swift; sourceTree = "<group>"; };
 		A5001101 /* Assets.xcassets */ = {isa = PBXFileReference; lastKnownFileType = folder.assetcatalog; path = Assets.xcassets; sourceTree = "<group>"; };
+		IC000002 /* AppIcon.icon */ = {isa = PBXFileReference; lastKnownFileType = folder; path = AppIcon.icon; sourceTree = "<group>"; };
 		B2E7294509CC42FE9191870E /* xterm-ghostty */ = {isa = PBXFileReference; lastKnownFileType = file; path = "ghostty/terminfo/78/xterm-ghostty"; sourceTree = "<group>"; };
 		C1ADE00001A1B2C3D4E5F719 /* claude */ = {isa = PBXFileReference; lastKnownFileType = text.script.sh; path = "Resources/bin/claude"; sourceTree = SOURCE_ROOT; };
 		D1BEF00001A1B2C3D4E5F719 /* open */ = {isa = PBXFileReference; lastKnownFileType = text.script.sh; path = "Resources/bin/open"; sourceTree = SOURCE_ROOT; };
@@ -328,6 +329,7 @@
 				B9000003A1B2C3D4E5F60719 /* CLI */,
 				087C454FFF74443AB06942C3 /* Resources */,
 				A5001101 /* Assets.xcassets */,
+				IC000002 /* AppIcon.icon */,
 				A5001016 /* GhosttyKit.xcframework */,
 				A5001017 /* ghostty.h */,
 				A5001018 /* cmux-Bridging-Header.h */,

design/cmux.icon/icon.json

@@ -0,0 +1,35 @@
+{
+  "fill" : "automatic",
+  "groups" : [
+    {
+      "layers" : [
+        {
+          "glass" : false,
+          "image-name" : "cmux-icon-chevron 2.png",
+          "name" : "cmux-icon-chevron 2",
+          "position" : {
+            "scale" : 1,
+            "translation-in-points" : [
+              37.357790031201375,
+              -0.5
+            ]
+          }
+        }
+      ],
+      "shadow" : {
+        "kind" : "neutral",
+        "opacity" : 0.5
+      },
+      "translucency" : {
+        "enabled" : true,
+        "value" : 0.5
+      }
+    }
+  ],
+  "supported-platforms" : {
+    "circles" : [
+      "watchOS"
+    ],
+    "squares" : "shared"
+  }
+}