test(labeling): add comprehensive tests for medium-priority utilities

Add extensive test coverage for medium-priority labeling utility modules:

- labeler-utils.test.js (50+ test cases)
  * Tests fetchLabelerRules() YAML parsing from GitHub API
  * Tests applyLabelerRules() file glob and branch pattern matching
  * Covers file-based and branch-based label application
  * Real-world LightSpeedWP workflow scenarios

- label-heuristics.test.js (60+ test cases)
  * Tests suggestLabelsFromContent() content-based detection
  * Bug/feature/documentation/refactor keyword detection
  * Priority detection (urgent, critical, low priority)
  * Area detection (security, performance)
  * Real-world issue and PR examples

- label-utils.test.js (40+ test cases)
  * Tests labelsToMarkdownTable() formatting utilities
  * Tests diffLabels() for comparing label sets
  * Edge cases (empty arrays, special characters, duplicates)
  * Integration scenarios for complete workflows

- check-template-labels.test.js (40+ test cases)
  * Tests template label validation against canonical labels
  * Label extraction from issue templates and issue-types.yml
  * Various label formats (brackets, quotes, inline)
  * Real-world template validation scenarios

Total: 190+ test cases added
Coverage improvement: Medium-priority files now at 100% (4/4 files)
Overall include coverage: 70% (16/23 files)

This brings total test coverage to production-ready standards for all
critical and medium-priority labeling agent utilities.

Author: claude