ls: contain PermissionError on stat and readdir

[ninowalker]

Discovered while developing. Companion to #10 and #11 — same family, found the same way, by hitting it.

The problem

ls catches FileNotFoundError on its operand's stat and PermissionError on readdir, but a backend whose stat raises PermissionError escapes bash.exec() as a Python exception. cat and stat fail the same backend in-band:

ls /denied        -> PermissionError escapes
cat /denied/x     -> exit 1, "cat: /denied/x: Permission denied"
stat /denied      -> exit 1, "stat: /denied: ..."

ls -l has the same gap on per-entry stat: a child whose stat refuses escapes mid-listing, where a vanished child already renders as ?????????.

The change

• Operand stat: except PermissionError alongside FileNotFoundError — ls: cannot access 'PATH': Permission denied, exit 2.
• Per-entry stat in the -l, -1 -F, and plain -F paths: PermissionError joins FileNotFoundError (placeholder row / skip), so one denied child no longer aborts the listing.
• readdir already caught PermissionError; untouched.

A sweep found this is not just ls

Running every non-network command against a backend that denies /denied on stat/readdir/read, the same gap is in 32 other commands (head, tail, wc, sort, md5sum, awk, find, du, tree, touch, bash, sh, ...). Those raise for the same reason ls did: the read/stat call catches FileNotFoundError but not PermissionError. #11 contains them all at the boundary; this PR gives ls the coreutils-consistent message the issue asked for. The others can take the same one-line treatment if you'd like them to.

Tests

tests/test_commands/test_minor_commands.py — a DenyingFs backend and four tests: denied operand (plain and -l) reports cannot access with exit 2, denied readdir stays in-band, and a denied child renders as ????????? without aborting the listing.

ruff check on the touched files: the 5 pre-existing findings in ls.py before and after; the test file is clean.