feat(cache): add child workspace isolation for multi-product CI builds (#777)

Implement two-level workspace isolation pattern for enterprise-scale CI:
- Atomic O(1) workspace restore via filesystem move (no tar/download/extract)
- Separate Library caching for independent restore
- .git preservation for delta operations
- Stale workspace cleanup with configurable retention policies
- 5 new action inputs: childWorkspacesEnabled, childWorkspaceName,
  childWorkspaceCacheRoot, childWorkspacePreserveGit,
  childWorkspaceSeparateLibrary
- 28 unit tests covering all service methods

This enables enterprise CI where workspaces are 50GB+ and traditional
caching via actions/cache is impractical. On NTFS, workspace restore
is O(1) via atomic rename when source and destination are on the same volume.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/model/build-parameters.ts

@@ -113,6 +113,11 @@ class BuildParameters {
   public localCacheRoot!: string;
   public localCacheLibrary!: boolean;
   public localCacheLfs!: boolean;
+  public childWorkspacesEnabled!: boolean;
+  public childWorkspaceName!: string;
+  public childWorkspaceCacheRoot!: string;
+  public childWorkspacePreserveGit!: boolean;
+  public childWorkspaceSeparateLibrary!: boolean;
   public lfsTransferAgent!: string;
   public lfsTransferAgentArgs!: string;
   public lfsStoragePaths!: string;
@@ -263,6 +268,11 @@ class BuildParameters {
       localCacheRoot: Input.localCacheRoot,
       localCacheLibrary: Input.localCacheLibrary,
       localCacheLfs: Input.localCacheLfs,
+      childWorkspacesEnabled: Input.childWorkspacesEnabled,
+      childWorkspaceName: Input.childWorkspaceName,
+      childWorkspaceCacheRoot: Input.childWorkspaceCacheRoot,
+      childWorkspacePreserveGit: Input.childWorkspacePreserveGit,
+      childWorkspaceSeparateLibrary: Input.childWorkspaceSeparateLibrary,
       lfsTransferAgent: Input.lfsTransferAgent,
       lfsTransferAgentArgs: Input.lfsTransferAgentArgs,
       lfsStoragePaths: Input.lfsStoragePaths,