Remove clone-swift.sh and CloneSwiftPlugin in favor of submodule

Swift headers are now provided via the Checkouts/swift git submodule,
so the manual clone script, build tool plugin, and CI action are no
longer needed.

Author: Kyle-Ye

.github/actions/checkout-swift-headers/action.yml

@@ -266,13 +266,6 @@ extension [SwiftSetting] {
     }
 }
 
-// MARK: - Plugin
-
-let swiftClonePlugin = Target.plugin(
-    name: "CloneSwiftPlugin",
-    capability: .buildTool()
-)
-
 // MARK: - Targets
 
 let platformTarget = Target.target(
@@ -304,7 +297,7 @@ let openAttributeGraphCxxTarget = Target.target(
     linkerSettings: [
         .linkedLibrary("z"),
     ],
-    plugins: [.plugin(name: swiftClonePlugin.name)]
+    plugins: []
 )
 let openAttributeGraphTarget = Target.target(
     name: "OpenAttributeGraph",
@@ -447,7 +440,6 @@ if computeCondition {
         package.targets.append(target)
     } else {
         package.targets.append(contentsOf: [
-            swiftClonePlugin,
             platformTarget,
             utilitiesTarget,
             openAttributeGraphTarget,

Package.resolved

@@ -64,14 +64,10 @@ The current suggested toolchain to build the project is Swift 6.1.2 / Xcode 16.4
 
 ### Clone Swift headers
 
-The project requires Swift toolchain headers for compilation. You can either clone them manually or let the build plugin handle it:
+The project requires Swift toolchain headers for compilation. They are included as a git submodule:
 
 ```shell
-# Option 1: Clone headers manually
-./Scripts/clone-swift.sh
-
-# Option 2: Let the build plugin clone headers (requires --disable-sandbox)
-swift build --disable-sandbox
+git submodule update --init
 ```
 
 ## License