chore: add pull request template

[griffinmartin]

Summary

Adds .github/pull_request_template.md to give contributors a standardized structure for describing changes and to remind them of the required CI checks before requesting review.

The template uses the repo's existing make conventions (make all wraps lint + build + test, matching Makefile:39) and links to Conventional Commits since semantic-pr.yml already enforces that on PR titles.

Related issue

N/A

Testing

• Ran `make all` locally — lint clean, build clean, 211/211 tests passing
• Verified file path matches GitHub's documented convention for auto-population

Checklist

• PR title follows Conventional Commits (`feat:`, `fix:`, `docs:`, `chore:`, etc.)
• `make all` passes locally (runs lint, build, and test)
• Tests added or updated where applicable (N/A — docs-only change)
• README or docs updated where applicable (N/A — this is the new doc)