chore: rename repository for discoverability

[zongqichen]

Summary

• rename the canonical GitHub repository to cloud-foundry-cli-contexts
• update repository, release, security, workflow, and Agent Skill URLs
• preserve the cfs executable and legacy Go module/install path for compatibility
• document and test the split between canonical repository URLs and the stable Go module path

Closes #23

Validation

• make check
• make release-check
• make security
• make smoke — not applicable; shim discovery, environment handling, execution, and isolation are unchanged
• Old GitHub and Git remotes redirect to the canonical repository
• go install github.com/zongqichen/cfs/cmd/cfs@v0.3.0 succeeds after the rename
• The migrated update endpoint reports the canonical v0.3.0 release URL

Checklist

• Tests and documentation cover the behavior change.
• 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.