docs+ci: strengthen PR template (expected results) + enforce in CI

[lassebenni]

What I built

Adds a package-complete PR template plus CI enforcement to this assignment repo.

• .github/PULL_REQUEST_TEMPLATE.md: tailored to this week's deliverables, with sections What I built / How to review / How to run / What reviewers should see (expected results) / Known limitations / Self-check. Gives a reviewing mentor a package-complete, reproducible submission.
• .github/workflows/pr-body-check.yml: fails a PR whose description is missing the required headings. GitHub only auto-injects the template in the web compose form / gh pr create without --body; a PR opened via the REST API or --body "..." (the path most AI tools take) skips it silently. This check closes that gap. Editing the PR description re-runs the check automatically (no new commit needed).

How to review

Two added files (one edited on Week 11). Read the template top-to-bottom; the workflow greps case-insensitively for 5 headings.

How to run

No runtime. Verified locally: the filled template passes the check; an empty/free-form body fails and lists the missing headings.

What reviewers should see (expected results)

• Filled template -> check passes (see this PR's own PR body check).
• Blob body -> check fails, lists missing sections.

Self-check

• Template + workflow validated locally against the check logic
• Workflow uses pull_request (not pull_request_target) with contents: read only
• No secrets introduced

🤖 Generated with Claude Code