feat(KB-044): add test coverage reporting with vitest coverage-v8

- Add @vitest/coverage-v8 to all workspace packages
- Configure vitest coverage settings in all vitest.config.ts files
- Align vitest versions across packages for consistency
- Add 'test:coverage' script to root package.json
- Verify all tests pass with coverage reporting enabled

.gitignore

@@ -5,6 +5,9 @@ node_modules/
 dist/
 *.tsbuildinfo
 
+# Coverage reports
+coverage/
+
 # hai runtime (local board state — don't commit)
 .kb/
 .hai/