docs(project): add stackctl CLI agent skill

[wax911]

Description

• Add the spec-compliant stackctl-cli Agent Skill under skills/stackctl-cli.
• Document source-of-truth and command-reference guidance to reduce CLI hallucinations.
• Add README installation steps for consumers using npx skills.
• Fix the Quick Start plan example and command-count wording.

Ref: AniTrend/homebrew-tap#22

Verification

• deno fmt --check README.md skills/stackctl-cli
• git diff --check
• GitNexus detect_changes(scope: staged), risk low, no affected processes
• Final Oracle review approved with low residual risk

[github-actions]

Auto approved automated PR