feat: add first-class install-cortex skill

[jmagar]

Summary

• add the first-class install-cortex guided installer skill
• align native/plugin setup, auth, deployment, and client verification guidance
• fix the setup adapter so OAuth preserves Cortex's secure default of disabling the static token unless dual mode is explicitly requested

Validation

• maintained Skill Creator validator/evaluator: 100/100, Grade A, low risk, 0 warnings/failures
• isolated Skills CLI install into a clean Codex home
• cargo test --test auth_modes: 16 passed, 0 failed
• installer contract tests: 4 passed
• Rust default/override auth tests passed
• shell syntax and git diff --check passed