Add pull request checklist template

[silentgeckoaudit3801]

Summary

• Adds .github/PULL_REQUEST_TEMPLATE.md so new PRs pre-populate with the requested test, formatting, lint, docs, and merge-marker checklist.
• Includes a verification section for commands/results and coverage evidence when contract logic is touched.

Closes #268.

Validation

• git diff --check
• Static marker check confirmed the template includes tests, cargo test, cargo fmt -- --check, cargo clippy -- -D warnings, docs, conflict markers, and verification prompts.

No project dependencies or toolchains were installed or executed; this is a repository metadata/docs-only change.

[1nonlypiece]

@silentgeckoaudit3801 this PR needs to be linked to an issue you're actually assigned to before it can be merged through the campaign — closing for now, happy to take another look once that's in place.