feat: self-host star history chart to fix broken embed

GitHub restricted detailed stargazer history to repo admins/collaborators,
which broke star-history.com's unauthenticated README embed.

- Add scripts/generate-star-history: Python script that fetches star data
  from GitHub API and renders a self-contained SVG chart
- Add .github/workflows/star-history.yml: weekly CI job that regenerates
  the chart using the workflow's GITHUB_TOKEN and commits the updated SVG
- Add docs/images/star-history.svg: initial chart generated from seed data
- Update docs/README.md: replace broken api.star-history.com embed with
  a reference to the self-hosted SVG

Closes #2040

Author: Copilot

.github/workflows/star-history.yml

@@ -0,0 +1,50 @@
+name: Star history chart
+
+## Regenerates the README's star-history chart. GitHub restricted
+## detailed stargazer history to repo admins/collaborators, which broke
+## star-history.com's unauthenticated embed — so we render the chart
+## ourselves with the repo-scoped workflow token and commit the SVG.
+on:
+  workflow_dispatch:
+  schedule:
+    ## Weekly, Monday 06:20 UTC — off the top of the hour to dodge the
+    ## scheduled-workflow load spike.
+    - cron: "20 6 * * 1"
+
+permissions:
+  contents: write
+
+jobs:
+  update-chart:
+    runs-on: ubuntu-latest
+    ## Scheduled runs on forks would fail (no stargazer access) and spam.
+    if: github.repository == 'commitizen-tools/commitizen'
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Generate chart
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+          ## Optional fallback: a collaborator PAT, only needed if GitHub
+          ## ever stops granting the workflow token stargazer access.
+          STAR_HISTORY_TOKEN: ${{ secrets.STAR_HISTORY_TOKEN }}
+        run: python scripts/generate-star-history --repo "$GITHUB_REPOSITORY"
+
+      - name: Commit updated chart
+        run: |
+          ## Stage first so a brand-new (untracked) chart is detected too —
+          ## `git diff --quiet` alone ignores untracked files.
+          git add docs/images/star-history.svg docs/images/star-history.json
+          if git diff --staged --quiet; then
+            echo "chart unchanged, nothing to commit"
+            exit 0
+          fi
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git commit -m "chore: update star history chart [skip ci]"
+          git push

docs/README.md

@@ -305,7 +305,13 @@ For more detailed information about argcomplete configuration and troubleshootin
 
 ## Star History
 
-[![Star History Chart](https://api.star-history.com/svg?repos=commitizen-tools/commitizen)](https://star-history.com/#commitizen-tools/commitizen)
+<!-- Regenerated weekly by .github/workflows/star-history.yml:
+     GitHub restricted stargazer history to repo collaborators, which broke
+     star-history.com's unauthenticated embed, so the chart is now rendered
+     in CI and committed here. -->
+<a href="https://github.com/commitizen-tools/commitizen/stargazers">
+  <img src="images/star-history.svg" alt="Star History Chart" width="700">
+</a>
 
 
 ## Sponsors