#2872 gatorwalk-factory: the swamp-club adapter links the pull request on the Lab issue, as issue-lifecycle's link_pr does

Found in the gatorwalk dogfood run on #2711 (PR #401).

What issue-lifecycle does

link_pr PATCHes the issue with githubPrUrl and githubPrNumber (extensions/models/_lib/swamp_club.ts, linkPr, about line 512). swamp-club applies it through issue.linkPr(), which adds a pr_linked issue event. The client's comment says this field "is what puts the PR link in the reporter's shipped notification". #2769 has both fields and the event.

What gatorwalk does

publish writes the pr_linked lifecycle entry, with the URL in its payload, and nothing else. swamp-club does not derive the issue's PR fields from that entry. #2711 shipped with no githubPrUrl, no githubPrNumber and no pr_linked event. The swamp-club adapter has no way to set them: nothing under gatorwalk-factory/ mentions githubPr. The mapping doc (swamp-club-swamp-extensions.md) marks link_pr as "Fits".

Fix direction

• Add a declared entry field alongside setsType, such as setsPr: url. When an entry declares it, publish PATCHes the issue's PR fields just before writing the entry, idempotently through the delivery ledger. The adapter writes the fields, so publish stays the only writer.
• Declare it on the pull-request stage's pr_linked entry in swamp-club-swamp-extensions.yaml.
• A new PR after merge.new-pr replaces the link, as issue-lifecycle's latest link_pr does.
• Fix the link_pr row in the mapping doc.

Done when

• Against the Lab fake, publishing pull-request evidence sets githubPrUrl and githubPrNumber once, and a re-run writes nothing.
• A second pull request replaces the link.
• The mapping row is accurate.