docs: add pull request template with testing checklist

[smoparth]

Add .github/PULL_REQUEST_TEMPLATE.md with concise What/Why sections and checklists for testing (hatch commands) and coding standards (type annotations, docstrings, commit format). The template serves as a lightweight reminder of CONTRIBUTING.md requirements at PR submission time.

Closes: #968

Summary by CodeRabbit

• Chores
  • Added a GitHub pull request template to standardize contribution submissions with predefined sections and guidelines checklist.

[tiran]

I have to admit that I don't like long PR templates with lots of check boxes. I find it annoying to check a bunch of checkboxes every time I create a PR.

What problems is the template addressing? Are we adding the PR template because we got bad PRs? Or are we adding a template just because some quality check tool wants it?

I would rather invest in better contribution documentation and automated checks. The template could be reduced to something like this:

## What

<!-- Brief description of the change. -->

## Why

<!-- Link to issue (Closes #NNN) or explain the motivation. -->

- [ ] PR is following [CONTRIBUTING.md](https://github.com/python-wheel-build/fromager/blob/main/CONTRIBUTING.md) guidelines

[tiran]

@Mergifyio rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[coderabbitai]

📝 Walkthrough

Walkthrough

Added a pull request template file at .github/PULL_REQUEST_TEMPLATE.md to standardize PR submissions with structured sections for describing changes and motivation, alongside a reminder to follow contribution guidelines.

Changes

Cohort / File(s)	Summary
Pull Request Template .github/PULL_REQUEST_TEMPLATE.md	New template file introducing predefined sections (What, Why) and checklist items to guide contributors through the PR submission process and reinforce adherence to CONTRIBUTING.md guidelines.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 A template appears, so neat and so bright,
To guide every PR through the review-day night,
What, Why, and a checklist—all set just right,
No more wandering PRs—they'll get it all right! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The PR description indicates the template was intentionally made minimal per maintainer preference, but the linked issue #968 specifies detailed testing and standards checklists. The implementation deviates from the original requirements.	Either update the template to include the full testing checklist (hatch commands) and standards checklist as specified in issue #968, or clarify why the minimal approach better serves the project's needs.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding a pull request template. It is concise, clear, and directly related to the changeset.
Out of Scope Changes check	✅ Passed	The pull request only adds the .github/PULL_REQUEST_TEMPLATE.md file, which is directly in scope with issue #968 requirements. No extraneous changes are present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[coderabbitai]

🧹 Nitpick comments (1)

.github/PULL_REQUEST_TEMPLATE.md (1)

11-11: Optional: Consider more active checkbox wording.

The current wording is clear, but you might consider a slightly more active phrasing to make it more personal to the contributor:

-- [ ] PR follows [CONTRIBUTING.md](https://github.com/python-wheel-build/fromager/blob/main/CONTRIBUTING.md) guidelines
+- [ ] I have read and followed the [CONTRIBUTING.md](https://github.com/python-wheel-build/fromager/blob/main/CONTRIBUTING.md) guidelines

This is purely stylistic—the current version is perfectly acceptable.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In @.github/PULL_REQUEST_TEMPLATE.md at line 11, The PR template checkbox "[ ]
PR follows
[CONTRIBUTING.md](https://github.com/python-wheel-build/fromager/blob/main/CONTRIBUTING.md)
guidelines" is phrased passively; update the checkbox text in
.github/PULL_REQUEST_TEMPLATE.md to a more active, contributor-focused wording
such as "I followed the CONTRIBUTING.md guidelines" or "I have followed the
CONTRIBUTING.md guidelines" so contributors affirm ownership when checking the
box—replace the existing checkbox line accordingly.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In @.github/PULL_REQUEST_TEMPLATE.md:
- Line 11: The PR template checkbox "[ ] PR follows
[CONTRIBUTING.md](https://github.com/python-wheel-build/fromager/blob/main/CONTRIBUTING.md)
guidelines" is phrased passively; update the checkbox text in
.github/PULL_REQUEST_TEMPLATE.md to a more active, contributor-focused wording
such as "I followed the CONTRIBUTING.md guidelines" or "I have followed the
CONTRIBUTING.md guidelines" so contributors affirm ownership when checking the
box—replace the existing checkbox line accordingly.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 1c69844a-955d-44f6-a57a-8a6e79f15d83

📥 Commits

Reviewing files that changed from the base of the PR and between 0635c76 and 4e9eb53.

📒 Files selected for processing (1)

• .github/PULL_REQUEST_TEMPLATE.md