chore(infra): remove git-subrepo, convert to remote branch-based dependencies

- Removed Packages/SundialKitStream and Packages/SundialKitCombine subrepo directories
- Updated Examples/Sundial/Package.swift to use remote GitHub URLs for plugins
- Plugin repositories now use branch-based dependencies (v1.0.0 for plugins, v2.0.0 for main package)
- Verified demo apps build successfully with remote dependencies

This completes the transition from monorepo + git-subrepo to multi-repository architecture with Swift Package Manager remote dependencies.

Documentation updates to follow in subsequent commit.

Author: leogdion

Examples/Sundial/Package.resolved

@@ -34,18 +34,18 @@ let package = Package(
     ),
     // SundialKit core (parent package)
     .package(
-      name: "SundialKit",
-      path: "../.."
+      url: "https://github.com/brightdigit/SundialKit.git",
+      branch: "v2.0.0"
     ),
-    // SundialKitCombine plugin (subrepo)
+    // SundialKitCombine plugin
     .package(
-      name: "SundialKitCombine",
-      path: "../../Packages/SundialKitCombine"
+      url: "https://github.com/brightdigit/SundialKitCombine.git",
+      branch: "v1.0.0"
     ),
-    // SundialKitStream plugin (subrepo)
+    // SundialKitStream plugin
     .package(
-      name: "SundialKitStream",
-      path: "../../Packages/SundialKitStream"
+      url: "https://github.com/brightdigit/SundialKitStream.git",
+      branch: "v1.0.0"
     )
   ],
   targets: [