chore: add issue and pull request templates

[Hollujay]

Summary

This repo takes outside contributions but gives contributors no structure for
filing an issue or opening a PR. Templates reduce round-trips by asking for the
right context up front.

Acceptance Criteria

• Add .github/ISSUE_TEMPLATE/ with at least a bug report and a feature
  request template
• Add .github/PULL_REQUEST_TEMPLATE.md prompting for: what changed, why,
  how it was tested, and whether it touches the trust boundary
• Reference CONTRIBUTING.md's explicit out-of-scope list in the feature
  request template, so proposals for slashing/disputes get redirected early

Tech Stack

Markdown, GitHub repo configuration