ENH: Add QuantEcon Style Guide Checker GitHub Action

[Copilot]

This PR implements a comprehensive GitHub Action for automated QuantEcon Style Guide compliance checking, addressing the need for consistent documentation quality across all QuantEcon repositories.

Overview

The new qe-style-check action provides intelligent, AI-enabled style checking with two operation modes:

1. PR Mode: Triggered by @qe-style-check mention in PR comments
2. Scheduled Mode: Weekly comprehensive reviews with optional individual PRs per lecture

Key Features

Intelligent Analysis

• 29 style rules covering writing conventions, code style, mathematical notation, and figure guidelines
• 3 confidence levels: High (auto-commit), Medium (review recommended), Low (human evaluation needed)
• Rule-based checking with numbered references (R001-R029) for easy tracking

Flexible Configuration

- uses: QuantEcon/meta/.github/actions/qe-style-check@main
  with:
    style-guide: '.github/copilot-qe-style-guide.md'  # Path or URL
    lectures: 'lectures'                               # Target directory
    exclude-files: 'index.md,references.md'          # Regex patterns
    confidence-threshold: 'high'                      # Auto-commit level
    create-individual-prs: 'true'                     # Scheduled mode option

Comprehensive Reporting

• Summary statistics with file and suggestion counts
• Detailed tables organized by confidence level
• Before/after examples for high confidence suggestions
• Rule references with line numbers for easy navigation

Style Rules Implemented

Writing Conventions (R001-R005)

• One-sentence paragraphs
• Proper heading capitalization
• Clear, concise language

Code Style (R006-R015)

• Unicode Greek letters (α instead of alpha)
• Modern timing patterns (qe.Timer() context manager)
• PEP8 compliance checking

Mathematical Notation (R016-R021)

• Proper transpose notation (\top instead of ^T)
• Built-in equation numbering
• Standard matrix/vector conventions

Figure Guidelines (R022-R029)

• No embedded matplotlib titles
• Proper line width settings (lw=2)
• Lowercase axis labels

Usage Examples

PR Style Check:
Comment @qe-style-check on any pull request to trigger style analysis of changed files.

Scheduled Review:

name: Weekly Style Check
on:
  schedule:
    - cron: '0 9 * * 1'  # Monday 9 AM UTC
jobs:
  style-check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: QuantEcon/meta/.github/actions/qe-style-check@main
        with:
          github-token: ${{ secrets.GITHUB_TOKEN }}
          mode: 'scheduled'
          create-individual-prs: 'true'

Testing

Comprehensive test suite includes:

• Local functionality tests (test/qe-style-check/test-basic.sh)
• Sample files with known style issues and clean examples
• GitHub Actions workflow tests for all modes
• File exclusion and confidence level validation

The action successfully detects style violations and provides actionable suggestions while maintaining compatibility with existing QuantEcon repository structures.

Fixes #226.

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[mmcky]

Will take a different approach to this action due to limitations in making suggestions in PRs (in the github interface).