cli: docref check <directory> fails with raw EISDIR instead of scanning the directory (or erroring clearly)

[PaulDotterer]

docref 0.1.0, Linux.

docref check [paths...] accepts file paths, but passing a directory surfaces a raw filesystem error:

$ docref check content/02-concepts
EISDIR: illegal operation on a directory, read

Expected either of:

1. (preferred) treat a directory argument as "all scanned docs under this directory" — the natural way to gate one section of a docs tree, and what every caller tries first; or
2. a clear usage error ("check takes file paths or globs, got directory 'content/02-concepts'").

Currently the only options are per-file invocations or a full-repo run. Agents and CI scripts pass directories reflexively; several of ours tripped on this in the same session.