test(read): isolate the rewrite test from the developer's rtk config

`rewritten_head_spellings_match_native_on_non_utf8_files` isolates
`CLAUDE_CONFIG_DIR` but reads the ambient `XDG_CONFIG_HOME`, so a config
carrying `exclude_commands = ["head", "tail"]` suppresses the rewrite and
the exit-code-3 assertion fails. Clean runners have no such config, so the
test passes in CI and fails only for the developers most likely to have
opted out of these very rewrites.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>