ci: switch update-snapshots workflow to fixtures repo (prep for #390) (#392)

* ci: switch update-snapshots workflow to fixtures repo

Lands the update-snapshots workflow change from #390 ahead of the rest of
the migration. issue_comment triggers always run the workflow file from
the default branch, so /update-new-snapshots on #390 was using the OLD
(lecture-python-programming) workflow against the NEW (fixture-path)
spec, producing 404-page screenshots. Merging this first lets #390's
/update-new-snapshots run correctly: clone fixtures at FIXTURES_SHA,
build them, run Playwright with the PR's spec, commit baselines back to
the PR branch.

This change is independent — it only affects /update-new-snapshots and
/update-snapshots PR-comment workflows, not ci.yml or any per-PR run.
ci.yml will follow in #390 along with the spec/tox/config changes that
depend on it.

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

* ci: clarify "keep in sync with ci.yml" comment

Per Copilot review: until #390 merges, ci.yml still builds
lecture-python-programming and has no FIXTURES_SHA, so a "keep this in
sync" cross-reference would send maintainers on a false trail. Reword to
acknowledge the transitional state — references #390 and explains the
sync becomes meaningful only after both workflows reference the fixtures
repo.

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

---------

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

Author: mmcky

.github/workflows/update-snapshots.yml

@@ -3,6 +3,14 @@ on:
   issue_comment:
     types: [created]
 
+# Snapshots are generated against quantecon-book-theme-fixtures at this
+# pinned SHA. ci.yml will be migrated to the same fixtures repo in #390
+# (it still builds lecture-python-programming on main today); once that
+# lands, keep FIXTURES_SHA in sync between the two workflows.
+env:
+  FIXTURES_REPO: QuantEcon/quantecon-book-theme-fixtures
+  FIXTURES_SHA: d8ffc17c753ecf45fa25c6062827e1aa9de201b3
+
 jobs:
   # /update-new-snapshots — only creates MISSING snapshots (safe for adding new tests)
   update-new-snapshots:
@@ -33,31 +41,27 @@ jobs:
           ref: ${{ steps.pr.outputs.ref }}
           token: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Fetch lecture-python-programming
-        shell: bash -l {0}
-        run: |
-          git clone --branch quantecon-book-theme https://github.com/QuantEcon/lecture-python-programming
+      - name: Checkout fixtures
+        uses: actions/checkout@v6
+        with:
+          repository: ${{ env.FIXTURES_REPO }}
+          ref: ${{ env.FIXTURES_SHA }}
+          path: fixtures
 
-      - name: Setup Anaconda
-        uses: conda-incubator/setup-miniconda@v4
+      - name: Setup Python
+        uses: actions/setup-python@v6
         with:
-          auto-update-conda: true
-          auto-activate-base: true
-          miniconda-version: 'latest'
           python-version: "3.13"
-          environment-file: lecture-python-programming/environment.yml
-          activate-environment: lecture-python-programming
+          cache: "pip"
 
-      - name: Install quantecon-book-theme
-        shell: bash -l {0}
+      - name: Install fixtures build deps + this PR's theme
         run: |
-          python -m pip install .
+          pip install -r fixtures/requirements.txt
+          pip install .
 
-      - name: Build HTML
-        shell: bash -l {0}
-        run: |
-          cd lecture-python-programming
-          jb build lectures --path-output ./
+      - name: Build fixtures site
+        working-directory: fixtures
+        run: jb build . --warningiserror
 
       - name: Setup Node.js
         uses: actions/setup-node@v6
@@ -74,7 +78,7 @@ jobs:
         continue-on-error: true
         run: npx playwright test --update-snapshots=missing
         env:
-          SITE_PATH: lecture-python-programming/_build/html
+          SITE_PATH: fixtures/_build/html
 
       - name: Commit and Push Updated Snapshots
         run: |
@@ -140,31 +144,27 @@ jobs:
           ref: ${{ steps.pr.outputs.ref }}
           token: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Fetch lecture-python-programming
-        shell: bash -l {0}
-        run: |
-          git clone --branch quantecon-book-theme https://github.com/QuantEcon/lecture-python-programming
+      - name: Checkout fixtures
+        uses: actions/checkout@v6
+        with:
+          repository: ${{ env.FIXTURES_REPO }}
+          ref: ${{ env.FIXTURES_SHA }}
+          path: fixtures
 
-      - name: Setup Anaconda
-        uses: conda-incubator/setup-miniconda@v4
+      - name: Setup Python
+        uses: actions/setup-python@v6
         with:
-          auto-update-conda: true
-          auto-activate-base: true
-          miniconda-version: 'latest'
           python-version: "3.13"
-          environment-file: lecture-python-programming/environment.yml
-          activate-environment: lecture-python-programming
+          cache: "pip"
 
-      - name: Install quantecon-book-theme
-        shell: bash -l {0}
+      - name: Install fixtures build deps + this PR's theme
         run: |
-          python -m pip install .
+          pip install -r fixtures/requirements.txt
+          pip install .
 
-      - name: Build HTML
-        shell: bash -l {0}
-        run: |
-          cd lecture-python-programming
-          jb build lectures --path-output ./
+      - name: Build fixtures site
+        working-directory: fixtures
+        run: jb build . --warningiserror
 
       - name: Setup Node.js
         uses: actions/setup-node@v6
@@ -181,7 +181,7 @@ jobs:
         continue-on-error: true
         run: npx playwright test --update-snapshots
         env:
-          SITE_PATH: lecture-python-programming/_build/html
+          SITE_PATH: fixtures/_build/html
 
       - name: Upload Old vs New Diff
         uses: actions/upload-artifact@v7