chore: add issue and pull request templates

[kleyt0n]

Problem

.github/ has no templates. CONTRIBUTING.md asks bug reporters to attach surface --check and surface --json --offline output, and gives contributors a four-command pre-PR checklist — none of which is enforced or even prompted. CODE_OF_CONDUCT.md is also missing.

What to do

• .github/ISSUE_TEMPLATE/bug_report.yml with surface --check and surface --json --offline output as required fields, plus OS, install method, and whether the build has default features.
• .github/ISSUE_TEMPLATE/feature_request.yml.
• .github/ISSUE_TEMPLATE/config.yml pointing vulnerability reports at SECURITY.md rather than the tracker, as README.md already instructs.
• .github/PULL_REQUEST_TEMPLATE.md mirroring the four commands.
• CODE_OF_CONDUCT.md.

Acceptance

A new bug report arrives with the diagnostic output already attached.