On a preview, the /__forge/ door links "What changed" to the pull's files, for a reader of the repository #7885

Intent: someone on a preview finds the diff it is a preview of in one tap from the forge door, as the P6 umbrella (#592) asks: "in a preview ... its diff".

Today the door (src/apps/door.ts) links the pull number in the where line and lists open pulls, but nothing leads to what this preview changes.

Done when:

• [ ] On a preview door, a reader who may read the repository (where.repo set) sees a "What changed" link to <forge>/<repo>/pulls/<n>/files, in the links list next to Home.
• [ ] A guest of the app (no where.repo) and a production door see no such link: nothing about the repository leaks to someone who cannot read it.
• [ ] The href is built with new URL from forgeUrl and escaped like the other links; the door's CSP and headers are unchanged.
• [ ] test/apps-door.test.ts (or a new test beside it) asserts the link on a preview for a reader, and its absence for a guest and on production; it fails without the change.

Files: src/apps/door.ts, test/apps-door.test.ts.

Risks: #7855 and #7853 also edit door.ts's links list; rebase on whichever lands first and keep the order Home, repository, What changed, Make it yours, Tell.

Suggested builder: scout (owns the door's recent work).