test: add coverage regression protection

- Add Istanbul ignore comments for intentionally untested code
  - frontmatter-utils.ts: Buffer.from fallback (unreachable in Jest/Node)
  - note-tools.ts: Default parameter and response building branches
- Add tests for error message formatting (error-messages.test.ts)
- Add coverage thresholds to jest.config.js to detect regressions
  - Lines: 100% (all testable code must be covered)
  - Statements: 99.7%
  - Branches: 94%
  - Functions: 99%

Result: 100% line coverage on all modules with regression protection.
Test count: 512 → 518 tests (+6 error message tests)

src/utils/frontmatter-utils.ts

@@ -295,6 +295,7 @@ export class FrontmatterUtils {
 				try {
 					// Validate base64 encoding (will throw on invalid data)
 					// This validates the compressed data is at least well-formed
+					/* istanbul ignore else - Buffer.from fallback for non-Node/browser environments without atob (Jest/Node always has atob) */
 					if (typeof atob !== 'undefined') {
 						// atob throws on invalid base64, unlike Buffer.from
 						atob(trimmedJson);