[Enhancement] Replace placeholder root README.md

[CTristan]

Summary

Replace the current one-line placeholder README ("Temporary holding spot for workflows until our full public reveal.") with real content describing this repo's purpose.

Why

This README describes the .github repo itself (not the org profile — that's profile/README.md tracked separately). Before public reveal, the placeholder wording is confusing for anyone who lands here directly.

Acceptance Criteria

• Root README.md explains this is the org-level .github repo
• Documents what lives here: reusable workflows, community health defaults, issue/PR templates, org profile
• Shows how consuming repos reference the reusable workflows (example uses: snippet)
• Links to profile/README.md for the org landing page
• Clarifies inheritance model (all-or-nothing per file) for contributors extending org defaults