Use project yamllint rules for changelog entry line lengths

[Copilot]

The hard-coded 136-character limit and whitespace heuristic diverge from yamllint’s configurable line-length rules. This change delegates validation to yamllint using the four-space indentation applied during YAML rendering.

• Validation
  • Remove the manual limit; report only line-length diagnostics with entry paths and line numbers.
• Configuration
  • Pass the project’s .yamllint path into the checker.
  • Cache configuration loading; support an explicit config and fall back to extends: default.
• Regression coverage
  • Cover indentation, multiple errors, non-breakable words/mappings, configuration loading, and unrelated-rule filtering.
  • Preserve real yamllint behavior: single-token references pass, but the supplied labeled :ref: example still produces a diagnostic because it contains a space.