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"

TEST_DATA.md

@@ -20,7 +20,7 @@ This is the primary test user used in most integration tests.
 
 **Used in:**
 - Go Tests: `cmd/server/tests/test_helpers.go` (getTestUserID, loginTestUser)
-- Bruno: `user/auth/Login User.bru`, `user/auth/Register User.bru`
+- Bruno: `user/auth/Login User.yml`, `user/auth/Register User.yml`
 
 **Notes:**
 - Automatically created if doesn't exist
@@ -58,7 +58,7 @@ Used for testing admin creation restrictions and multi-admin scenarios.
 ```
 
 **Used in:**
-- Bruno: `user/admin/Register Admin User.bru`
+- Bruno: `user/admin/Register Admin User.yml`
 
 ## Test Libraries