feat(F-15): citation graph — PageRank, coupling, co-citation, CLI
- codex/graph.py: build_citation_graph (DiGraph from DB), citation_pagerank
(graceful uniform fallback < 5 nodes), find_related (bibliographic coupling),
find_co_cited, dangling_citations
- codex/config.py: graph_cite_boost_alpha=0.3, graph_min_corpus_size=15
- codex/cli.py: graph report [--top-n] [--json], graph related <paper-id>
[--min-shared]; search paper --cite-boost (PageRank score weighting)
- tests/graph/: 39 tests (graph functions + CLI) — 326 total green
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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