Add Keito skill onboarding commands

[sionsmith]

Summary

• add keito skill install/status/doctor for installing and verifying the Agent Skill
• prompt after interactive keito auth login to install the skill
• keep JSON output clean by suppressing child installer output in JSON/quiet mode
• add integration coverage for fake npx skills installs and hook installers
• make PATH binary detection Windows-aware for tools like npx.cmd/jq.cmd
• pin the skills installer package to skills@1.5.6 by default and document the GitHub-sourced skill install path
• bump CLI release version to 0.1.5 with changelog entry

Verification

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• floating skills installer scan: no skills@latest or bare npx skills add references
• floating GitHub Actions scan: no workflow action refs using @v*, @main, @master, or @latest
• temporary-home install with local npx skills source and keito --json skill install --skip-skills-add