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"

PROJECT_GUIDELINES.md

@@ -128,7 +128,7 @@ VERIFY → Compile successfully
 - ✅ Stop building server binaries - everything goes through containers
 
 ### API Changes (Full-Stack Tasks Only)
-- ✅ Include **Bruno DSL .bru requests** with all API documentation
+- ✅ Include **Bruno OpenCollection YAML requests** with all API documentation
 - ✅ Tests must be **comprehensive and cover three contexts**: no user, user, and admin
 - ✅ Maintain backward compatibility for mobile apps and external consumers
 
@@ -204,7 +204,7 @@ VERIFY → Compile successfully
 | **Quick start/setup** | `README.md` | Installation, environment setup, first-run |
 | **Breaking changes** | Both `README.md` and relevant `docs/` | Migration guides, deprecation notices |
 | **Bug fixes** | Update relevant `docs/` only if user-visible | Clarifications, troubleshooting additions |
-| **Bruno API tests** | `.bru` files alongside code | API contract testing, examples |
+| **Bruno OpenCollection YAML tests** | `.yml` files in bruno folder in appropriate folder/sub-folder | API contract testing, examples |
 
 **Documentation Update Workflow**:
 1. **Identify the audience** (end users, developers, API consumers)
@@ -212,7 +212,7 @@ VERIFY → Compile successfully
 3. **Update documentation** before or with code changes
 4. **Verify documentation renders** at `/docs` endpoint
 5. **Test search** finds new/updated content
-6. **For API changes**: Update both `docs/developer/api/` files AND Bruno `.bru` files
+6. **For API changes**: Update both `docs/developer/api/` files AND Bruno OpenCollection YAML `.yml` files
 7. **Commit separately** with clear message: `docs: <description>`
 
 **When in doubt**:
@@ -312,7 +312,7 @@ git checkout -- internal/handlers/auth.go
   - [ ] New endpoints → Create new `.md` file in `docs/developer/api/`
   - [ ] API behavior → Update existing `docs/developer/api/` files
   - [ ] Breaking changes → Both `README.md` + relevant `docs/`
-  - [ ] Bruno `.bru` files → Update/create alongside API changes
+  - [ ] Bruno OpenCollection YAML `.yml` files → Update/create alongside API changes
 
 ### During Schema Changes (Full-Stack Only)
 - [ ] Read current schema completely
@@ -336,7 +336,7 @@ git checkout -- internal/handlers/auth.go
 - [ ] Verify database has new schema (check column types, indexes, etc.)
 
 ### After API Changes
-- [ ] Create/update Bruno DSL .bru requests
+- [ ] Create/update Bruno OpenCollection YAML requests
 - [ ] Test with no user context
 - [ ] Test with regular user context
 - [ ] Test with admin context
@@ -352,7 +352,7 @@ git checkout -- internal/handlers/auth.go
 - [ ] Verify logical commit structure
 - [ ] **Update documentation** (see Documentation section):
   - [ ] User-facing changes → `docs/user/`
-  - [ ] API changes → `docs/developer/api/` + Bruno `.bru` files
+  - [ ] API changes → `docs/developer/api/` + Bruno OpenCollection YAML `.yml` files
   - [ ] Setup/onboarding → `README.md`
   - [ ] Development changes → `docs/contributing/`
 - [ ] **Verify docs render** at `/docs` endpoint