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/brainy.ts

@@ -2178,7 +2178,18 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       // Step 2: Copy storage ref (COW layer - instant!)
       await refManager.copyRef(currentBranch, branchName)
 
-      // Step 2: Create new Brainy instance pointing to fork branch
+      // CRITICAL FIX (v5.3.6): Verify branch was actually created to prevent silent failures
+      // Without this check, fork() could complete successfully but branch wouldn't exist,
+      // causing subsequent checkout() calls to fail (see Workshop bug report).
+      const verifyBranch = await refManager.getRef(branchName)
+      if (!verifyBranch) {
+        throw new Error(
+          `Fork failed: Branch '${branchName}' was not created. ` +
+          `This indicates a storage adapter configuration issue. Please report this bug.`
+        )
+      }
+
+      // Step 3: Create new Brainy instance pointing to fork branch
       const forkConfig = {
         ...this.config,
         storage: {