Governance: Add PR and issue templates

[evans-sam]

Problem

No PR template. No issue templates. Contributors get no structured guidance when filing.

Why it matters

Structured issues and PRs save maintainer triage time by front-loading the right questions. They also let issue types be categorized automatically.

Proposed approach

1. Add .github/PULL_REQUEST_TEMPLATE.md:
   • Summary
   • Related issue(s)
   • Type of change (bug/feature/docs/refactor)
   • Testing done
   • Breaking changes
   • Checklist (tests added, docs updated, CHANGELOG noted)
2. Add .github/ISSUE_TEMPLATE/:
   • bug_report.yml (version, reproduction steps, expected/actual, environment)
   • feature_request.yml (problem, proposed solution, alternatives)
   • config.yml with contact_links pointing to SECURITY.md for vulns and (optional) Discussions for questions.

Acceptance criteria

• Issue-create flow shows a template chooser with bug + feature options.
• PR-create flow pre-fills the template.
• Security link routes to SECURITY.md, not the issue tracker.