chore: add .gitignore and PR #37 finalization plan

- .gitignore: protect docs/comparison-results/ and other private
  benchmark artifacts from accidental upstream push
- docs/plans: add plan for PR #37 Codex auth finalization

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

.gitignore

@@ -0,0 +1,16 @@
+# Private benchmark / evaluation artifacts — never push to upstream
+docs/comparison-results/
+scripts/evaluate-synthesis.py
+scripts/generate-synthesis-inputs.py
+fixtures/polymarket_sample.json
+docs/v2.1-tweets.md
+docs/30-day-anniversary-thread.md
+docs/30-day-anniversary-tweets.md
+variants/open/references/research.md
+
+# OS / tool files
+.DS_Store
+.claude/
+.entire/
+__pycache__/
+*.pyc