Contributor program: good-first-issues, Discussions, templates

[AKogut]

Context

Lower the barrier for external contributors.

Scope

• Curate a set of good first issue tasks across packages
• Seed Discussions (Q&A, Ideas, Show & Tell) with starter threads
• Polish CONTRIBUTING, issue/PR templates, PR checklist (changeset required)
• Development guide: local setup, architecture map, testing conventions

Acceptance criteria

• A first-time contributor can pick a task and open a PR without asking how to set up