refactor: inline cache clearing into restore action

Consolidates cache clearing and restore into a single self-contained
action, eliminating the need for a separate clear-cache action.

Changes:
- Inline cache clearing logic directly into xahau-ga-cache-restore
- Delete xahau-ga-clear-cache action (no longer needed)
- Add additional-clear-keys input for clearing multiple keys
- Remove nd-experiment-overlayfs branch from workflow triggers

Benefits:
- Single action does everything (clear + restore)
- Simpler architecture (one action instead of two)
- Self-contained and easier to maintain

.github/workflows/xahau-ga-nix.yml

@@ -2,7 +2,7 @@ name: Nix - GA Runner
 
 on:
   push:
-    branches: ["dev", "candidate", "release", "nd-experiment-overlayfs-2025-10-29"]
+    branches: ["dev", "candidate", "release"]
   pull_request:
     branches: ["dev", "candidate", "release"]
   schedule: