fix(F-15): address Opus review findings (CRITICAL + WARN)

CRITICAL:
- cli.py: invert cite-boost formula: divide distance by (1 + alpha*pr)
  instead of multiply — high-PageRank papers now correctly rank higher
- cli.py: wire graph_min_corpus_size config into graph report; warning
  emitted to stderr so JSON stdout stays parseable

WARN:
- graph.py: document that damping is ignored in small-graph uniform branch
- cli.py: sort dangling citations before slicing for stable output
- cli.py: replace raise typer.Exit(0) with return in empty-graph path
- tests: fix *extra_args helper signatures; add cite-boost ordering test
  and small-corpus warning test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

codex/graph.py

@@ -64,9 +64,13 @@ def citation_pagerank(graph: nx.DiGraph, *, damping: float = 0.85) -> dict[str,
     -------
     ``{paper_id: score}`` dict.  Scores sum to approximately 1.0.
 
-    When ``graph`` has fewer than :data:`_MIN_PAGERANK_NODES` nodes,
-    returns a uniform distribution and logs a warning — the signal is
-    too thin to be meaningful.
+    When ``graph`` has fewer than :data:`_MIN_PAGERANK_NODES` (5) nodes,
+    returns a uniform distribution and logs a warning — the graph is too
+    sparse for the random-walk model to converge meaningfully.  The
+    ``damping`` parameter is ignored in this branch.  The user-facing
+    corpus-size threshold (default 15) lives in
+    :attr:`codex.config.Settings.graph_min_corpus_size` and is surfaced
+    by ``codex graph report``.
     """
     n = graph.number_of_nodes()
     if n == 0: