ci: Add git-town workflow to display PR stack visualization

[ericapisani]

Add a GitHub Actions workflow that uses git-town/action to automatically render the stacked pull request visualization in PR descriptions.

When a PR is opened, reopened, or pushed to, the workflow injects the branch stack into the <!-- branch-stack --> marker in the PR template. This makes it easy to see the full stack context without leaving the PR page.

The PR template is updated to include the marker and a "Pull request stack" section where the visualization will appear — always below the user-provided description.

The workflow follows existing repo security conventions: both actions are pinned to full commit SHAs, permissions are deny-all at the workflow level with only contents: read and pull-requests: write granted at the job level, and fetch-depth: 0 is set so git-town has the full history needed to compute the stack.

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

This is how your changes will appear in the changelog.
Entries from this PR are highlighted with a left border (blockquote style).

---

Internal Changes 🔧

• Add git-town workflow to display PR stack visualization by ericapisani in #5577

---

_{🤖 This preview updates automatically when you update the PR.}

[github-actions]

Codecov Results 📊

✅ 134 passed | Total: 134 | Pass Rate: 100% | Execution Time: 21.50s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +108
Passed Tests	📈 +108
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 13215 uncovered lines.
✅ Project coverage is 33.59%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    28.02%    33.59%    +5.57%
==========================================
  Files          189       189         —
  Lines        19899     19899         —
  Branches      6460      6460         —
==========================================
+ Hits          5575      6684     +1109
- Misses       14324     13215     -1109
- Partials       525       659      +134

---

Generated by Codecov Action