chore: add PR and issue templates

[jose-compu]

Why

When someone opens a PR there is no template guiding them on what to include. This leads to inconsistent PR descriptions and makes review harder.

What to add

Create .github/PULL_REQUEST_TEMPLATE.md with sections like:

• What — brief description of the change
• Why — motivation / linked issue
• How — implementation approach
• Checklist:
  • Tests added/updated
  • forge fmt passes
  • NatSpec complete for new/modified public functions
  • CHANGELOG.md updated (if applicable)
  • No new compiler warnings

Also consider adding issue templates (.github/ISSUE_TEMPLATE/) for bug reports and feature requests. The default GitHub ones are fine as a starting point.