test(editor): rename unit test files from phase-based to module-descriptive names

Replace sprint/audit-based naming with names that describe what
the tests actually cover:

- auditFixes.test.ts → headingStructureValidation.test.ts
- sprintFixes.test.ts → schemaAndPluginIntegrity.test.ts
- backlogFixes.test.ts → inputRuleAndPastePosition.test.ts
- dataSafetyFixes.test.ts → clipboardSafetyGuards.test.ts

No test logic changes. Improves discoverability when searching
for tests by feature area.

Made-with: Cursor

Author: HMarzban