Add librarian Stage 1 oversize check

Third Stage 1 piece: scripts/librarian_check_oversize.py walks
non-archive markdown files and flags any over the 500-line soft cap
declared in AGENTS.md. Wired into scripts/librarian.sh.

Caught one real finding on first run: docs/performance.md at 914
lines. Splitting it into sub-topic notes is a separate cleanup task
— surfaced for the user, not auto-applied.

Updates docs/plans/librarian.md Progress + sets the next concrete
step to a stale-plan checker.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/librarian.sh

@@ -21,6 +21,12 @@ if ! uv run python scripts/librarian_check_citations.py; then
 fi
 echo
 
+echo "→ File size (500-line soft cap from AGENTS.md)"
+if ! uv run python scripts/librarian_check_oversize.py; then
+    overall=1
+fi
+echo
+
 if [ "$overall" -eq 0 ]; then
     echo "All Stage 1 checks passed."
 else