docs: update Bruno terminology to OpenCollection YAML format

Update all references from "Bruno DSL .bru files" to "Bruno OpenCollection YAML .yml files" to reflect the current Bruno format. This includes:

- PROJECT_GUIDELINES.md: Update API testing requirements
- README.md: Update command examples
- TEST_DATA.md: Update test data references
- docs/contributing/development.md: Update API testing section
- docs/developer/api-reference.md: Update Bruno testing documentation
- docs/developer/collections-api.md: Update test file references
- scripts/README.md: Update validation script documentation
- scripts/verify-guidelines.sh: Update file extension check (.bru → .yml)
- bruno/opencollection.yml: Rename collection from "Untitled Collection" to "Bookhoard"

docs/developer/api-reference.md

@@ -1606,10 +1606,10 @@ bruno/
 ├── sync-kobo/              # Kobo sync protocol
 ├── conflicts/               # Conflict resolution
 ├── queue/                   # Sync queue management
-└── collection.bru           # Main collection file
+└── collection.yml           # Main collection file
 ```
 
-## Testing with Bruno
+## Testing with Bruno OpenCollection YAML
 
 Install Bruno CLI:
 ```bash