feat: add optional cfs Agent Skill

[zongqichen]

Summary

• add a canonical optional Agent Skill for safe workspace-default and named-context routing
• document explicit Codex and Claude Code installation without changing global agent settings
• add a credential-free real Codex discovery and behavior smoke test

Closes #12.

Validation

• make check
• make release-check
• make security
• CFS_REAL_CF=/home/d072944/bin/cf make smoke
• CFS_REAL_CF=/home/d072944/bin/cf make e2e
• Agent Skill structural validator
• make agent-smoke with real Codex CLI
• isolated Claude Code skill smoke using claude-opus-4-8
• Claude Code Opus 4.8 review and post-fix re-review: no actionable findings

Checklist

• Tests and documentation cover the behavior change.
• Notable changes are 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.