docs: clarify EverOS former name

[cyfyifanchen]

Summary

• Update the public EverMem use-case copy to identify the current product as EverOS while retaining formerly called EverMemOS for historical context.
• Keep the English README, Chinese README, and both use-case documentation mirrors aligned without changing historical project URLs or compatibility identifiers.

Area

• Architecture method
• Benchmark
• Use case
• Documentation
• Developer experience
• CI, build, or release

Verification

make docs-check
make ci
make check-cjk  # advisory; reports 10 existing repo-wide hits outside the changed locale mirror
make check-commits RANGE=origin/main..HEAD
make check-pr-title PR_TITLE='docs: clarify EverOS former name'

Checklist

• I kept the change scoped to the relevant area.
• I am opening this from a separate branch, not pushing directly to main.
• I updated docs, examples, or setup notes when behavior changed.
• I added or updated tests when the change affects behavior.
• I did not commit secrets, .env files, dependency folders, or generated output.
• Active relative links in Markdown files resolve.

Notes for Reviewers

Historical external repository URLs and demo compatibility identifiers still contain EverMemOS intentionally. Renaming those would break links or interfaces; this PR only updates the product name shown on the main repository's public documentation surfaces.

By submitting this pull request, I agree that my contribution is licensed under
the Apache License 2.0.