A directory's `?all=1` lists at most 5,000 entries and says how many it left out (from branch scout/6196-…) #6439

Later: The repository is flow only (#5930, owner, 2026-10-26); this directory ceiling is feature work for after the alarms stay clear.

Replaces #6196. The work is built on branch scout/6196-a-directory-s-all-1-lists-at-most-5-000 (pull #6211). The review found it right on its code: 5,000 entries, directories first, a count of what is left, the default window unchanged, and the README updated.

Done when:

• [ ] That branch is rebased on main and opened as a pull, still waiting for #6145 if that is open.
• [ ] The directory tests it touches pass, and README.md's "Big directories" names the 5,000 ceiling.