Add methodology registry and AI-powered PR review workflow

[igerber]

• Add docs/methodology/REGISTRY.md documenting academic foundations and implementation requirements for all 12 estimators
• Add .github/workflows/ai_pr_review.yml for automated PR review using OpenAI Codex, triggered on PR open or /ai-review comment
• Add .github/codex/prompts/pr_review.md with review prompt that prioritizes methodology adherence over code quality
• Add .github/pull_request_template.md requiring contributors to document methodology references and intentional deviations

[github-actions]

Overall assessment: ⚠️ Needs changes

Executive summary

• Documentation: TOC anchor typo breaks the TwoWayFixedEffects link in the new methodology registry
• Methodology: No estimator/method changes in this PR, so no registry cross-check required beyond the new doc itself
• Workflow: AI review workflow appears appropriately scoped and gated; no security issues found

Methodology

• No findings. No estimator/math/SE/default behavior changes in this PR.

Code Quality

• No findings.

Performance

• No findings.

Maintainability

• No findings.

Tech Debt

• No findings.

Security

• No findings. The workflow appropriately gates /ai-review to trusted roles and uses OPENAI_API_KEY from secrets.

Documentation/Tests

• Severity: P3
  Impact: The Table of Contents link for TwoWayFixedEffects is broken due to a misspelled anchor, reducing navigability of the methodology registry.
  Concrete fix: Correct the anchor in the TOC to #twowayfixedeffects to match the heading ID.
  Location: docs/methodology/REGISTRY.md:L12-L20

[igerber]

/ai-review