The forge toolbar on a preview names its pull and links to it #10763

Intent: never lost: from a preview you can always get back to the change it shows.

Done when:

• [ ] First confirm in src/apps/toolbar.ts whether the toolbar on a preview already names its pull; if so, close with the commits named.
• [ ] On a preview host (--<pull>--) the toolbar's first line reads "Preview of pull #N" and links to the pull page; on production it is unchanged.
• [ ] The number is parsed and validated at the boundary (digits only) and the text escaped.
• [ ] A test renders the toolbar for a production and a preview host and asserts both.

Files: src/apps/toolbar.ts, its test. Risks: toolbar is injected into every app; keep the added bytes small. Suggest: scout.

Done on main already, waiting for you to close it: 2afe4c137cd4d2538710a03b76d6d9ccce0bab7b c17a132b989db3fb1bb4b26bb980cc5c6baadf3d

Closed: already done on main in 2afe4c1, c17a132. Closed by the crew's live triage; reopen it to undo, and it stays open.