Make ck self-contained via PEP 723 inline script dependencies

[rex1fernando]

Summary

• Adds a PEP 723 inline script metadata block to ck declaring its runtime dependencies, and changes the shebang to #!/usr/bin/env -S uv run --script, so a plain ./ck works with no venv or pip install step — uv resolves and caches the dependencies on first run. pipx run ./ck works as well (pipx parses PEP 723 natively).
• Updates the README: setup is now “install uv, put ck on your PATH” instead of the venv wrapper shell function, and the testing commands use uv run --with-requirements requirements.txt so they work without a venv.
• Pins bibtexparser<2 in both the inline block and requirements.txt: the code uses the v1 API (bibtexparser.bwriter, .customization, .latexenc), which was removed in bibtexparser 2.x, so an unpinned install can break.

Notes:

• The explicit --script flag in the shebang is required because the file has no .py extension, so uv doesn't auto-detect the inline metadata (uv run ./ck alone would skip it).
• weasyprint (in requirements.txt) is not in the inline block because it isn't imported anywhere in the code; pytest is test-only.
• The old venv + requirements.txt workflow keeps working — for that path, invoke the script as python3 ck.
• Running through a symlink from PATH still resolves the local citationkeys package (Python resolves the symlink when setting sys.path[0]).

Test plan

• ./ck --help runs from a clean environment (verified: uv installs the 9 deps into a cached env, CLI works)
• Same through a symlink from another directory
• pipx run ./ck --help (verified)
• Full pytest suite unaffected: 108 passed, 2 xfailed

🤖 Generated with Claude Code