Match the account name as a whole word in /doc-review's scrub

[mattmenefee]

Summary

• /doc-review's pre-publish scrub matched the account name from id -un as a bare substring, so any word containing it — and a public GitHub handle that begins with it — blocked a clean post. It fails closed, so nothing leaked, but a gate that routinely blocks harmless text teaches whoever runs it to work around it
• The name now has to stand alone, bounded by anything other than a letter or digit. That still catches a home-directory path, the dash-encoded form in per-project directory names, an email address and the bare name in prose
• The name is escaped before it enters the pattern, so an account name containing . or another regex metacharacter matches only itself
• Found while posting the review for PR Make /doc-review record findings honestly and publish them safely #123, where the substring form blocked comments on words like "matters" and "formatting"

Test plan

• markdownlint-cli with the repository's .markdown-lint.yml passes on doc-review.md
• Pattern block extracted from the command and run under bash and zsh against a corpus: with the old pattern, ordinary words and the public handle were flagged; with the new one they are not, while a home path, a dash-encoded path, the bare name, an email address and a line consisting only of the name are all still flagged
• Escaping checked with a dotted account name: john.doe is matched in a path and the look-alike johnxdoe is not
• CI lint passes