feat(cli): Add `papi bibtex` command for citation export

[hummat]

Problem or Motivation

No easy way to export BibTeX for LaTeX workflows. Users need to manually construct citations or copy from external sources. This complements existing papi add --from-file library.bib (import) to complete the import/export story.

Proposed Solution

Add papi bibtex command:

# Print to stdout
papi bibtex paper1 paper2

# Export to file
papi bibtex paper1 paper2 --to library.bib

# Export all papers
papi bibtex --all --to library.bib

# Customize citation keys
papi bibtex paper1 --key-style doi

Options

• PAPER...: Paper names (positional, multiple allowed)
• --all: Export all papers in the database
• --to FILE: Write to file instead of stdout (append if exists)
• --key-style {name,doi,arxiv,slug}: BibTeX key format
  • name: Use paperpipe name (default)
  • doi: Use DOI (e.g., 10.1234/example)
  • arxiv: Use arXiv ID (e.g., 2301.12345)
  • slug: Auto-generate from first author + year + title

Implementation

Complexity: LOW — metadata already stored, just formatting

1. Read meta.json for each paper
2. Format as BibTeX entries:
   • @article or @misc based on venue
   • Required fields: author, title, year
   • Optional: journal, doi, arxiv, url, abstract
3. Handle missing metadata gracefully (e.g., no DOI → use arXiv URL)
4. Ensure valid BibTeX (escape special chars, wrap long lines)

Example output

@article{lora,
  author = {Hu, Edward J. and Shen, Yelong and Wallis, Phillip and ...},
  title = {LoRA: Low-Rank Adaptation of Large Language Models},
  year = {2021},
  journal = {arXiv preprint arXiv:2106.09685},
  arxiv = {2106.09685},
  url = {https://arxiv.org/abs/2106.09685}
}

Alternatives Considered

• Export via papi show --format bibtex — conflates display with export, awkward for multiple papers
• External tools (e.g., Zotero) — adds dependency, doesn't use paperpipe metadata

Area

CLI commands