[Tier 2] Enable GitHub Discussions and link in README

[justsml]

Problem

GitHub Discussions is not enabled or not linked. Issues are the wrong venue for questions, ideas, and show-and-tell. Without a dedicated community space, users will open issues for support requests, polluting the bug tracker.

Goal

Enable GitHub Discussions as the primary community hub and link it prominently.

Acceptance Criteria

• Enable GitHub Discussions in repo settings (Settings → General → Discussions)
• Create a welcome/announcement discussion post
• Create discussion categories: General, Q&A, Ideas, Show and tell
• Update README to link to Discussions for questions and ideas
• Update CONTRIBUTING.md to direct general questions to Discussions instead of Issues
• Add a Discussions badge to README
• Pin a “How to ask a good question” guidelines post

Context

• README already has a Community section that can be expanded
• CONTRIBUTING.md should clarify the boundary between Issues (bugs/features) and Discussions (questions/ideas)

Labels

infrastructure, tier-2-growth-accelerator, good first issue