Add issue templates and PR template

[aarontrowbridge]

Adding standard open-source issue templates and PR template to Piccolo.jl (CONTRIBUTING.md already exists).

Acceptance Criteria

• .github/ISSUE_TEMPLATE/bug_report.yml — structured bug report
• .github/ISSUE_TEMPLATE/feature_request.yml — structured feature request
• .github/ISSUE_TEMPLATE/config.yml — redirect blank issues
• .github/pull_request_template.md — required issue reference, verification steps