docs(project): add operational playbooks to stackctl CLI skill

[wax911]

Description

• Add references/playbooks.md with task-oriented operational workflows for agentic systems.
• Cover setup, deploy, reload, drift check, teardown, secrets, env scaffolding, diagnostics, and preview playbooks.
• Include a decision tree mapping agent intents to commands, exit code interpretation, and safety patterns.
• Update SKILL.md with an operational model section and references to the new playbook file.

Verification

• deno fmt --check skills/stackctl-cli/ passes (4 files checked)
• Zero em dashes in all skill files
• All workflows verified against src/cli/mod.ts, src/compose/reload.ts, src/compose/sync.ts, and src/secrets/mod.ts

[github-actions]

Auto approved automated PR