Migrating from LH Response Handler

LH URL Redirects replaces LH Response Handler, which kept each rule as a post on the main site with the full matching URL in meta and the status code in a taxonomy. The two can run side by side during the switch, since both only act on 404s.

1. Network-activate LH URL Redirects.
2. Run lh-url-redirects/import-legacy-redirects with no arguments for a dry run, and read its per-rule report.
3. Run it again with dry_run false. Each published Response post becomes an exact rule, with its title as the note and its post ID kept as legacy_id, so a rerun skips what is already imported. Two posts that differ only by a trailing slash collapse into one rule.
4. Check a few sources with resolve-url and in a browser, then network-deactivate LH Response Handler.

The importer reads but never changes the old posts, so they remain as a backup until deleted.