refactor: remove OverlayFS delta caching entirely

THE GREAT CULLING: Remove all OverlayFS and delta caching logic.

After extensive investigation and testing, we determined that OverlayFS
file-level layering is fundamentally incompatible with ccache's access
patterns:

- ccache opens files with O_RDWR → kernel must provide writable file handle
- OverlayFS must copy files to upper layer immediately (can't wait)
- Even with metacopy=on, metadata-only files still appear in upper layer
- Result: ~366MB deltas instead of tiny incremental diffs

The fundamental constraint: cannot have all three of:
1. Read-only lower layer (for base sharing)
2. Writable file handles (for O_RDWR)
3. Minimal deltas (for efficient caching)

Changes:
- Removed all OverlayFS mounting/unmounting logic
- Removed workspace and registry tracking
- Removed delta creation and restoration
- Removed use-deltas parameter
- Simplified to direct tar/extract workflow

Before: 726 lines across cache actions
After:  321 lines (-55% reduction)

Benefits:
- ✅ Simpler architecture (direct tar/extract)
- ✅ More maintainable (less code, less complexity)
- ✅ More reliable (fewer moving parts)
- ✅ Same performance (base-only was already used)
- ✅ Clear path forward (restic/borg for future optimization)

Current state works great:
- Build times: 20-30 min → 2-5 min (80% improvement)
- Cache sizes: ~323-609 MB per branch (with zst compression)
- S3 costs: acceptable for current volume

If bandwidth costs become problematic, migrate to restic/borg for
chunk-level deduplication (completely different architecture).

.github/actions/xahau-ga-build/action.yml

@@ -76,7 +76,6 @@ runs:
           ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ inputs.main_branch }}
           ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-
           ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-
-        use-deltas: 'false'  # Disabled: OverlayFS file-level deltas incompatible with ccache's byte-level LRU updates
         aws-access-key-id: ${{ inputs.aws-access-key-id }}
         aws-secret-access-key: ${{ inputs.aws-secret-access-key }}
 
@@ -194,6 +193,5 @@ runs:
       with:
         path: ~/.ccache
         key: ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ steps.safe-branch.outputs.name }}
-        use-deltas: 'false'  # Disabled: OverlayFS file-level deltas incompatible with ccache's byte-level LRU updates
         aws-access-key-id: ${{ inputs.aws-access-key-id }}
         aws-secret-access-key: ${{ inputs.aws-secret-access-key }}