Add pull request template

[reinaldyalt]

Summary

Adds a .github/PULL_REQUEST_TEMPLATE.md so the PR form is auto-populated with the sections CONTRIBUTING.md requires. Fixes #565.

Changes

• New file .github/PULL_REQUEST_TEMPLATE.md with sections: Summary, Changes, Testing, Security considerations, Breaking Changes, Checklist
• Sections mirror the PR description requirements in CONTRIBUTING.md ("Pull request process" step 5)
• Security checklist items drawn from SECURITY.md (auth checks, arithmetic safety, gas/WASM size)
• Checklist reminds contributors to update docs/CHANGELOG.md and not disclose vulnerabilities publicly

No code, docs, or other files touched — template-only change.

Testing

• Verified the file renders correctly as markdown
• Confirmed GitHub auto-populates this file for new PRs against main (standard .github/PULL_REQUEST_TEMPLATE.md behavior)

No contract code changed, so cargo build/cargo test are unaffected.

Security considerations

None — this is a markdown-only template. No contract logic, storage, or auth paths touched.

Breaking Changes

None