feat: mosaic cite DOI — fetch and format citation strings #19

Summary

Add a mosaic cite DOI command that fetches metadata for a DOI and prints a formatted citation string in common styles (APA, MLA, BibTeX, etc.).

Motivation

Researchers frequently need quick citation strings while writing. A mosaic cite command would surface this directly in the terminal without opening a browser.

Proposed behaviour

# Default: BibTeX output
mosaic cite 10.48550/arXiv.1706.03762

# Specify citation style
mosaic cite 10.48550/arXiv.1706.03762 --style apa
mosaic cite 10.48550/arXiv.1706.03762 --style mla
mosaic cite 10.48550/arXiv.1706.03762 --style chicago
mosaic cite 10.48550/arXiv.1706.03762 --style bibtex   # default

# Copy to clipboard (requires pyperclip or xclip)
mosaic cite 10.48550/arXiv.1706.03762 --copy

Implementation notes

• Metadata source: Check local SQLite cache first; fall back to Crossref (api.crossref.org/works/{doi})
• BibTeX: Format from Paper fields — already well supported by Crossref
• Other styles: Use the Crossref content negotiation endpoint:

  GET https://doi.org/{doi}
  Accept: text/x-bibliography; style=apa
  

  This returns a pre-formatted string for any CSL style, no local rendering needed.
• Clipboard: Optional --copy flag using pyperclip (if installed) or xclip/pbcopy subprocess

Priority

Tier 2 — high utility, low complexity