fix: resolve fork() silent failure on cloud storage adapters

Fixed critical bug where fork() completed without errors but branches
were not persisted to storage, causing checkout() to fail with
"Branch does not exist" errors.

Root Cause:
- COW metadata paths (_cow/*) were being branch-scoped incorrectly
- resolveBranchPath() applied branch prefixes to COW paths
- Result: refs written to branches/main/_cow/... instead of _cow/...
- COW metadata (refs, commits, blobs) must be global, not per-branch

Changes:
1. baseStorage.ts (resolveBranchPath):
   - Bypass branch scoping for _cow/ paths
   - COW metadata now stored globally as designed
   - Fixes fork() persistence across all storage adapters

2. brainy.ts (fork):
   - Add branch creation verification after copyRef()
   - Throw descriptive error if branch wasn't created
   - Prevents silent failures in production

3. tests/integration/fork-persistence.test.ts:
   - Comprehensive integration tests for fork workflow
   - Tests: persist → listBranches → checkout
   - Covers Workshop snapshot use case
   - Verifies COW metadata is globally accessible

Impact:
- Affects: FileSystem, GCS, R2, S3, Azure storage adapters
- Workshop snapshot restoration now works
- Zero breaking changes, production-scale ready

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

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

src/storage/baseStorage.ts

@@ -362,6 +362,14 @@ export abstract class BaseStorage extends BaseStorageAdapter {
    * @protected - Available to subclasses for COW implementation
    */
   protected resolveBranchPath(basePath: string, branch?: string): string {
+    // CRITICAL FIX (v5.3.6): COW metadata (_cow/*) must NEVER be branch-scoped
+    // Refs, commits, and blobs are global metadata with their own internal branching.
+    // Branch-scoping COW paths causes fork() to write refs to wrong locations,
+    // leading to "Branch does not exist" errors on checkout (see Workshop bug report).
+    if (basePath.startsWith('_cow/')) {
+      return basePath  // COW metadata is global across all branches
+    }
+
     if (!this.cowEnabled) {
       return basePath  // COW disabled, use direct path
     }