The boards' flow panel lists the hot files: each file 3 or more open pulls touch, with its count, and the ones at the build limit marked #6420

Flow work for #5930 (the /flow page's "hot files, a bar per file"). src/crew/hot_files.ts already knows which files open pulls touch and holds builds on a file HOT_PULLS or more pulls touch, but no page shows it. The owner learns of a pile-up only from the tick line.

Done when:

• [ ] The flow panel (src/web/pages/flow-panel.ts, reached from /flow, which redirects to /boards) draws "Hot files": up to 10 files, most-touched first, each with its count of open pulls and a bar. A file at or over HOT_PULLS reads "builds held".
• [ ] It reads only what hot_files.ts keeps in memory, and never runs git on a page request. With nothing learned yet it says "No file is touched by 3 or more open pulls".
• [ ] Private repositories' files are shown only to a reader who may read that repository.
• [ ] A test seeds the cache with 12 files and checks the order, the cap of 10, the "builds held" mark and the empty line. The panel's existing tests still pass.

Files: src/web/pages/flow-panel.ts, src/crew/hot_files.ts (a read-only export, if one is needed), a new test/flow-hot-files.test.ts.