Add contributor scaffolding: issue templates, PR template, CONTRIBUTING.md

[max-wells]

Problem

Repo has no .github/ISSUE_TEMPLATE/, no PR template, no CONTRIBUTING.md. First-time contributors have no guided way to file bugs/features or open PRs.

Expected

• .github/ISSUE_TEMPLATE/bug_report.md and .github/ISSUE_TEMPLATE/feature_request.md (or a config.yml + forms).
• .github/PULL_REQUEST_TEMPLATE.md with a short checklist (tests, changelog entry per AGENTS.md).
• CONTRIBUTING.md at repo root pointing to AGENTS.md conventions (domains/shared structure, changelog entries, pnpm check:quality, git hooks via pnpm setup:hooks).

Scope

• .github/ISSUE_TEMPLATE/*
• .github/PULL_REQUEST_TEMPLATE.md
• CONTRIBUTING.md