docs: improve discovery for parallel CF sessions

[zongqichen]

Summary

• lead with the multiple-session and CF_HOME collision problem in the README
• add a factual comparison of manual CF_HOME, cf-targets-plugin, cfctx, and cfs
• document the cmd/cfs package for pkg.go.dev
• update GitHub description and topics with natural high-intent terms

Closes #29

Validation

• make check
• make release-check
• Comparison reference links return HTTP 200
• Security-sensitive changes: not applicable
• Shim behavior changes: not applicable

Checklist

• Documentation covers the intended distinction without unsupported superiority claims.
• The change is recorded under Unreleased in CHANGELOG.md.
• No credentials, tokens, CF configuration, or sensitive logs are included.
• The change is focused and does not publish a release.