feat: add CommonMark ipynb export and CI workflow

- Add `markdown: commonmark` to all 24 lecture export configs so ipynb
  exports produce plain CommonMark markdown cells compatible with
  vanilla Jupyter Notebook, JupyterLab, and Google Colab
- Add build-ipynb.yml workflow that clones QuantEcon/mystmd@myst-to-ipynb,
  builds from source, exports all ipynb files, and audits for MyST leaks
- All 24 notebooks pass audit (0 MyST syntax leaks)

Uses QuantEcon/mystmd myst-to-ipynb branch which adds:
- CommonMark AST pre-transform (admonitions, math, figures, exercises, etc.)
- Identifier/label stripping to prevent (label)= prefixes
- Image directive stripping for plain ![alt](url) output
- Empty cell filtering and block marker removal

Author: mmcky

.github/workflows/build-ipynb.yml

@@ -0,0 +1,79 @@
+name: Build ipynb [using QuantEcon/mystmd]
+on: [pull_request]
+jobs:
+  build-ipynb:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22.x
+
+      - name: Install Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+          cache: 'pip'
+          cache-dependency-path: 'myst_requirements.txt'
+
+      - name: Install execution requirements
+        run: python -m pip install -r myst_requirements.txt
+
+      - name: Clone QuantEcon/mystmd (myst-to-ipynb branch)
+        run: |
+          git clone --depth 1 --branch myst-to-ipynb https://github.com/QuantEcon/mystmd.git /tmp/mystmd
+
+      - name: Build mystmd from source
+        working-directory: /tmp/mystmd
+        run: |
+          npm ci
+          npx turbo run build
+
+      - name: Build HTML with custom mystmd
+        working-directory: ./lectures
+        run: /tmp/mystmd/packages/mystmd/dist/myst.cjs build --html
+
+      - name: Build ipynb exports with custom mystmd
+        working-directory: ./lectures
+        run: /tmp/mystmd/packages/mystmd/dist/myst.cjs build --ipynb
+
+      - name: Audit notebooks for MyST syntax leaks
+        working-directory: ./lectures
+        run: |
+          python3 -c "
+          import json, glob, re, sys
+          PATTERNS = [
+              (r'^\s*:::', 'directive fence'),
+              (r'\{math\}\`', 'math role'),
+              (r'^\+\+\+', 'block marker'),
+              (r'\{doc\}\`', 'doc role'),
+              (r'\{index\}', 'index directive'),
+              (r'\`\`\`\{', 'fenced directive'),
+              (r'^\([a-z_][a-z0-9_-]*\)=\$', 'target label'),
+          ]
+          issues = 0
+          for nb_path in sorted(glob.glob('exports/*.ipynb')):
+              with open(nb_path) as f:
+                  nb = json.load(f)
+              for i, cell in enumerate(nb.get('cells', []), 1):
+                  if cell.get('cell_type') != 'markdown':
+                      continue
+                  source = ''.join(cell.get('source', []))
+                  for line in source.split('\n'):
+                      for pat, desc in PATTERNS:
+                          if re.search(pat, line.strip()):
+                              print(f'{nb_path} cell {i}: [{desc}] {line.strip()[:80]}')
+                              issues += 1
+          if issues:
+              print(f'\n{issues} MyST syntax leaks found!')
+              sys.exit(1)
+          else:
+              print(f'All {len(glob.glob(\"exports/*.ipynb\"))} notebooks clean.')
+          "
+
+      - name: Upload ipynb exports
+        uses: actions/upload-artifact@v4
+        with:
+          name: ipynb-exports
+          path: './lectures/exports/'

lectures/about_py.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/about_py.ipynb
 downloads:
   - file: ./about_py.md

lectures/debugging.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/debugging.ipynb
 downloads:
   - file: ./debugging.md

lectures/functions.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/functions.ipynb
 downloads:
   - file: ./functions.md

lectures/getting_started.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/getting_started.ipynb
 downloads:
   - file: ./getting_started.md

lectures/intro.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/intro.ipynb
 downloads:
   - file: ./intro.md

lectures/jax_intro.md

@@ -11,6 +11,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/jax_intro.ipynb
 downloads:
   - file: ./jax_intro.md

lectures/matplotlib.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/matplotlib.ipynb
 downloads:
   - file: ./matplotlib.md

lectures/names.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/names.ipynb
 downloads:
   - file: ./names.md

lectures/need_for_speed.md

@@ -9,6 +9,7 @@ kernelspec:
   name: python3
 exports:
   - format: ipynb
+    markdown: commonmark
     output: exports/need_for_speed.ipynb
 downloads:
   - file: ./need_for_speed.md