Add librarian Stage 1 orchestrator (scripts/librarian.sh)

Single entry point that runs every Stage 1 check in order and
aggregates exit codes — both checks always run so users see all
findings in one pass. Two checks wired up today (lychee link
integrity, file:line citations), more land incrementally as Stage 1
grows.

Removes scripts/librarian.sh from the ignore list now that the file
exists. Updates docs/plans/librarian.md Progress + Next Step.

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

scripts/librarian-ignore.txt

@@ -12,9 +12,6 @@
 #
 # Comments after `#` are stripped per line. Blank lines ignored.
 
-# Future deliverables described in docs/plans/librarian.md
-scripts/librarian.sh
-
 # Future config + script described in docs/plans/model_size_experiment.md
 configs/deep_cfr/model-size-*.yaml
 scripts/run_model_size_experiment.sh