Pull request template is nearly empty and doesn't reflect the PR checklist in CONTRIBUTING.md

[codeforester]

Problem

.github/pull_request_template.md contains only empty section headers with no guidance:

## Summary

-

## Issue

Closes #

## Validation

-

## Demo Impact

None.

## Notes

None.

Meanwhile, CONTRIBUTING.md has a detailed PR checklist:

• The branch name follows <category>/<issue>-<YYYYMMDD>-<slug>.
• The PR is scoped to one issue...
• The PR body explains what changed and how it was validated.
• Relevant BATS and Python tests pass.
• Documentation is updated when behavior or user-facing commands change.
• The PR includes Fixes #<issue> when it should close the issue.
• Demo Impact is meaningful for needs-demo work, or explicitly says None.

The checklist exists but is invisible at PR creation time because it lives in a prose document, not in the template. Contributors opening PRs see empty sections with no hints about what to fill in.

Fix

Add brief inline guidance to each section of the template, and add the checklist as a checkbox section so contributors can verify each point before submitting:

## Summary

<!-- What changed and why. Focus on decisions, not a restatement of the diff. -->

## Issue

Closes #

## Validation

<!-- Commands run and their output. At minimum: `basectl test base`. -->

## Demo Impact

<!-- None. / Or: what changed and where the demo should be updated. -->

## Checklist

- [ ] Branch name follows `<category>/<issue>-<YYYYMMDD>-<slug>`
- [ ] PR is scoped to one issue
- [ ] Relevant BATS and Python tests pass
- [ ] Docs updated if user-facing behavior changed
- [ ] `Fixes #<issue>` in the body when this PR should close the issue