Use yamllint for changelog entry line-length checks

[Copilot]

The hard-coded 136-character limit approximates YAML indentation but misses yamllint’s exemption semantics. Delegate validation to the project’s .yamllint configuration.

• Validation
  • Prefix entry lines with four spaces and report only line-length problems, preserving entry line numbers and yamllint messages.
• Configuration
  • Pass the project path into the changes checker.
  • Support an explicit config override; fall back to yamllint defaults when .yamllint is absent.
• Regression coverage
  • Cover boundaries, multiline diagnostics, non-breakable references, config precedence, custom limits, and disabled rules.
  • Distinguish exempt single-token references from the supplied two-token :ref: example, which real yamllint rejects even inside a block scalar.