Every public app links to Report a problem: /__forge/report on the app host, a Link header, the scaffold footer #701

Pillar: P5 Open to agents (umbrella #591). One run, from today's main. Follows #677.

Outcome. Every public app hosted by the forge tells a visitor, person or agent, how to report a problem, with no change to the app itself for agents and a one-line change for people.

Acceptance.

• [ ] The app proxy answers /__forge/report on every public app's host with a redirect to the repository's /:owner/:name/report?from=<the app page>; a private app answers as today.
• [ ] HTML responses from a public app carry Link: <…/__forge/report>; rel="…" (a registered relation, chosen in the pull with its reason).
• [ ] The app scaffold (src/apps/scaffold.ts) renders a small "Report a problem" link to /__forge/report in its footer.
• [ ] Test: a public app's page has the header and /__forge/report redirects with the page; a private app's does neither.
• [ ] After merge, an issue on cheffryjames/cheffry-s-dungeon asks for the same footer link.

Done means reachable: the link on a scaffolded app's page, the header for agents, test, a phone and desktop look of a public app with the link.