docs: correct README facts and tidy prose

[lesnik512]

Summary

README fixes for facts that did not match the code, plus prose cleanup.

Changes

• A file containing only newlines is truncated to empty, not to one newline (eof_fixer/fixer.py:61-63, confirmed by running on a \n\n\n file: result is 0 bytes). Added to the list and the examples table.
• Exit code 2 was undocumented: argparse usage errors and a non-directory path (eof_fixer/main.py:22-23) exit 2, confirmed by running on a file and with no argument. Documented, and Usage now says the path must be a directory.
• The empty-file table row used empty inline code, which renders as literal backticks.
• uv add makes eof-fixer a project dependency; Installation now shows uv tool install eof-fixer and uvx eof-fixer ..
• License was listed twice; kept the footer link.
• Exit-code text: a second run after fixing exits 0 (confirmed). Dropped the pre-commit gate claim since the repo ships no pre-commit hook.
• Sentence-case headings, plain numbered list, no dashes, short contributing note instead of the boilerplate.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging