docs: refine target discovery terms

[zongqichen]

Summary

• cover both multiple sessions and multiple targets in the README and comparison heading
• use the common CF_HOME conflicts wording in user-facing metadata
• improve Agent Skill discovery for parallel CF CLI workflows without changing its safety rules
• update the GitHub description to match these high-intent searches

Closes #31

Validation

• make check
• make release-check
• make security
• make agent-smoke
• CFS_REAL_CF=/usr/bin/cf make e2e
• GitHub repository search now returns cfs for multiple targets and CF_HOME conflicts queries

Checklist

• Agent safety behavior is unchanged.
• The existing Unreleased changelog entry covers the refinement.
• No credentials, tokens, CF configuration, or sensitive logs are included.
• The change is focused and does not publish a release.