feat: add reference property to context.report() for external docs links

[mvanhorn]

Summary

Adds a reference property to context.report() so custom rules can link to external documentation explaining the design decision behind a rule.

Usage

report({
  message: 'Operation must include a request body',
  reference: 'https://wiki.example.com/api-guidelines#request-bodies',
});

Output (stylish format):

Operation must include a request body

Reference: https://wiki.example.com/api-guidelines#request-bodies

Changes

• packages/core/src/walk.ts: Added reference?: string to Problem and NormalizedProblem types
• packages/core/src/format/format.ts: Display reference URL in both stylish and GitHub Actions output formatters

The property passes through automatically via the ...opts spread in the normalization logic, so no additional wiring is needed.

Fixes #1052

This contribution was developed with AI assistance (Claude Code).

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[tatomyr]

Could you add a E2E or a unit test that demonstrates how the output will look like? And please do not modify our PR template.