chore(github): add issue-linked pull request template

[egdev6]

Linked issue

Closes #24

Summary

• Add the default .github/PULL_REQUEST_TEMPLATE.md with an explicit issue reference.
• Explain Closes for completed work and Refs for partial work, including the default-branch merge requirement.
• Request a short summary, observed verification results, and an evidence-backed checklist.

CI enforcement, package contents, and release behavior are intentionally unchanged. This template is a reminder, not an enforced gate.

Verification

• Linux, Node v24.20.0.
• npm test: passed package validation and 15/15 tests.
• npm run pack:check: passed with the expected 8 entries; the template is not packaged.
• git diff --cached --check: passed before commit.
• Markdown diagnostics and structural readback: clean.
• No RED/GREEN cycle: this is a passive Markdown template with no executable behavior. Modified scripts and skills: none; shellcheck and skill-load testing are not applicable.

Checklist

• The issue reference is correct: Closes for completed work or Refs for partial work.
• The scope is focused and preserves the skill safety contracts.
• Verification results and limitations are recorded above.
• Documentation is updated where applicable.

Issue #24 is approved; the PR has exactly one type label, type:chore.