recheck: a Markdown summary format and a one-run readability change table #3205

Problem

CI jobs that run redocly recheck post a summary comment on the pull request.
Today a workflow script builds that comment from --summary json and from two readability runs, one on the merge base and one on the head, so the comment can show the readability change per file.
That script lives in the workflow (#3187) and in the Redocly monorepo, so each repository copies it.

Proposal

1. Add --format=markdown to recheck, modeled on the lint format: a summary by rule with error, warning, and info counts, ready to post as a comment.
2. Let --readability take a git ref, for example --compare-to origin/main.
   For each file in the run, the command reads the base version with git show <ref>:<path>, scores both versions, and reports base, head, and change per file.
   One run then replaces the two worktree runs, and the Markdown format renders the table.

Reference

• chore: lint the docs and cookbook Markdown with redocly recheck #3187: the docs recheck job and the review thread that asked for this.
• The monorepo workflow .github/workflows/recheck.yml, which carries the same script.