Permission loader: a settings file that cannot exist, or holds only a BOM, fails closed #4651

With #4287, load_permission_rules adds a catch-all Ask when a Claude Code settings file exists but cannot be read or parsed, and skips missing or empty files. Two inputs fall on the wrong side of that line. Neither file holds any rules, but both get the Ask:

• The parent path is not a directory. If a project contains a regular file named .claude, user_dirs::project_root still accepts it as the marker, because it only checks .exists(). The same happens if CLAUDE_CONFIG_DIR points at a file. Reading <that>/settings.json then fails with ENOTDIR (ErrorKind::NotADirectory), not NotFound, so it is treated as unreadable. Windows reports NotFound for the same layout and skips the file, so the result depends on the platform.
• The file holds only a UTF-8 BOM. For example, a settings.local.json saved empty by an editor that writes EF BB BF. content.trim() does not remove U+FEFF, so the empty-file check misses it. from_json_str then strips the BOM, fails on the empty input, and the file counts as unparseable.

Measured with rtk rewrite "git status", where the other settings file has allow: ["Bash(git status)"]:

Under RTK_REWRITE_HOST=openclaw, a command no rule matches also goes from 0 to 3 in both cases.

What would close it:

• treat NotADirectory like NotFound (stable since Rust 1.83; the MSRV is 1.91);
• run the empty check after strip_leading_bom.

A test for each that goes through load_permission_rules_from.