fix: replace non-existent swift-action with container-based Swift setup

Replace swiftlang/swift-action@v2 (which doesn't exist) with the
container-based approach using the official swift:6.2-noble Docker image.

This matches the proven pattern from CelestraCloud's workflow and
eliminates the "Unable to resolve action swiftlang/swift-action,
repository not found" error.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: leogdion

.github/workflows/cloudkit-sync.yml

@@ -22,6 +22,7 @@ jobs:
   sync:
     name: Sync to CloudKit
     runs-on: ubuntu-latest
+    container: swift:6.2-noble
     timeout-minutes: 30
 
     permissions:
@@ -31,11 +32,6 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      - name: Setup Swift 6.2
-        uses: swiftlang/swift-action@v2
-        with:
-          swift-version: '6.2'
-
       - name: Verify Swift version
         run: |
           swift --version