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"

scripts/README.md

@@ -259,22 +259,22 @@ templates/... (truncated, first 10 shown)
 - `make verify-quick`: Runs critical-only validation
 - Fixed naming confusion (previously ran quick version for both targets)
 
-### Bruno API Tests Validation:
-- Counts Bruno `.bru` test files in `bruno/` directory
+### Bruno OpenCollection YAML Validation:
+- Counts Bruno OpenCollection YAML `.yml` test files in `bruno/` directory
 - Compares against API documentation file count in `docs/api/`
 - Flags insufficient coverage for human review
 - Ensures API documentation and test coverage stay synchronized
 
-## Bruno API Tests Explained
+## Bruno OpenCollection YAML Explained
 
 ### Purpose:
-Bruno v3.0 is an API testing tool that uses `.bru` files to define:
+Bruno is an API testing tool that uses OpenCollection YAML format (`.yml` files) to define:
 - API requests and responses
 - Test cases and validation
 - Contract testing examples
 
 ### Verification Logic:
-1. **Counts Test Files**: Scans `bruno/` directory for `.bru` files
+1. **Counts Test Files**: Scans `bruno/` directory for `.yml` files
 2. **Counts API Docs**: Scans `docs/api/` for `.md` files  
 3. **Compares Coverage**: Ensures test files >= documentation files
 4. **Flags Gaps**: Human review for insufficient test coverage
@@ -282,7 +282,7 @@ Bruno v3.0 is an API testing tool that uses `.bru` files to define:
 ### Example Scenario:
 ```
 Developer adds: docs/api/users/login.md
-Should also add: bruno/user/auth/Login User.bru
+Should also add: bruno/user/auth/Login User.yml
 Verification catches: API docs exist but no test file
 ```
 

scripts/verify-guidelines.sh

@@ -477,8 +477,8 @@ fi
 ###############################################################################
 section "Documentation: Bruno API Tests"
 
-echo "Checking Bruno API test files..."
-BRUNO_FILES=$(find bruno -name "*.bru" 2>/dev/null | wc -l)
+echo "Checking Bruno OpenCollection YAML test files..."
+BRUNO_FILES=$(find bruno -name "*.yml" 2>/dev/null | wc -l)
 if [ "$BRUNO_FILES" -gt 0 ]; then
     success_msg "Found $BRUNO_FILES Bruno test files"
 else