docs: add EverMind ecosystem overview

[cyfyifanchen]

Summary

• Add an early README section that frames EverMind as an open-source ecosystem.
• Add a reusable ecosystem table linking EverOS, EverAlgo, HyperMem, EverMemBench, EvoAgentBench, MSA, EverMe, evermem-claude-code, and everos-plugins.
• Position EverMemOS as the paper/research line carrying the strongest memory-system benchmark runs on top of EverOS.

Validation

• make docs-check
• uv run pre-commit run --all-files
• Checked every new GitHub repo URL returns HTTP 200

Notes

I verified the public EverMind-AI organization repo list before writing the table. I did not add a separate HyperMemBench link because no public repo by that name was visible under the org; the README instead describes HyperMem as its own benchmark-backed method.