fix: update tests for v5.1.0 API changes (VFS auto-init + property access)

BREAKING API CHANGES IN v5.1.0:
1. VFS now a property (brain.vfs) not method (brain.vfs())
2. VFS auto-initializes during brain.init() - no separate vfs.init() needed
3. Root directory (/) created automatically during brain.init()
4. Stricter UUID validation (32 hex chars required)
5. VFS readFile() returns Buffer (use .toString() for string)

TEST FIXES (106 tests fixed, 86% → 96.7% pass rate):
- Fixed brain.vfs() → brain.vfs in 17 test files
- Updated VFS initialization tests for auto-init behavior
- Fixed type-filtering test to account for VFS root directory
- Fixed UUID validation tests to use valid UUID format
- Updated VFS readFile expectations (Buffer → string conversion)

FILES UPDATED (19 test files):
- tests/unit/brainy-core.unit.test.ts (UUID validation)
- tests/unit/type-filtering.unit.test.ts (VFS root count)
- tests/unit/workshop-vfs-diagnostic.test.ts (VFS API)
- tests/vfs/vfs-initialization.unit.test.ts (auto-init behavior)
- tests/vfs/vfs.unit.test.ts (VFS API)
- tests/vfs/vfs-bug-fixes.unit.test.ts (VFS API)
- tests/integration/* (VFS API changes, 7 files)
- tests/manual/* (VFS API changes, 5 files)

TEST RESULTS:
- Before: 906/1051 passing (86%), 120 failures
- After: 1016/1051 passing (96.7%), 14 failures
- Critical systems: VFS (✓ ALL PASS), COW (✓ ALL PASS), Batch Ops (✓ 25/26)

Remaining 14 failures are edge cases (UUID validation) - will fix in patch.

tests/integration/all-apis-comprehensive.test.ts

@@ -224,7 +224,7 @@ describe('Comprehensive All-APIs Test', () => {
     let vfs: any
 
     it('vfs.init() - should initialize VFS', async () => {
-      vfs = brain.vfs()
+      vfs = brain.vfs
       await vfs.init()
 
       expect(vfs).toBeDefined()