fix: default-explain flags, check unreadable files, GitContext cwd

[hc-ui]

Summary

Hardening pass for eoldoctor 0.1.4. No framework change.

Fixes

• Bare eoldoctor --json / eoldoctor <path> now run explain with those args (the default-command path used to drop every extra argument or reject a path as an invalid subcommand).
• check fails on unreadable files (was info-only; CI gate returned 0).
• GitContext.list_files resolves paths against the context cwd so the library API works when the process cwd is not the repo root.
• Boolean git config still normalizes 1/yes/on on Git < 2.40 (fallback when --type=bool-or-str is missing).
• attributes --write reports I/O errors and exits 1 instead of a traceback.

Tests / lint / docs

• New unit tests for the above; compileall runs inside the existing unittest CI job.
• [tool.ruff] configured for local ruff check (bug-oriented rules).
• README CI example installs from git (not yet on PyPI); pre-commit rev pinned to v0.1.4.

Workflow files under .github/workflows/ were not modified (token has no workflow scope).

Verification

• python -m ruff check eoldoctor tests
• python -m unittest discover -s tests -t . — 164 tests OK locally
• python -m eoldoctor check --require lf --forbid-bom --require-final-newline on this repo: OK