chore: upgrade core to 2.3.0

[luojiyin1987]

Summary

• Bump the CLI version from 2.2.3 to 2.2.4.
• Set the minimum @lint-md/core version to ^2.3.0.
• Use fixMarkdown() for stdin and worker fix paths.
• Require an explicit fix mode for lint workers.
• Add regression tests for the API migration and opt-in rules.
• Document the new rules in both README files.
• Record the release changes in the changelog.

Impact

The three new rules remain disabled by default. Existing CLI output does not change.

Validation

• npm run lint
• npm test -- --runInBand
• npm run test:package