[codex] add AI skill usage disclaimer

[ktwu01]

What changed

• add an explicit AI Usage Disclaimer section to the manual skill template
• inject the same disclaimer into skill drafts generated from the extract-knowhow HTML report
• document in the READMEs that skills can trigger broad modifications, long workflows, and significant token usage
• add regression coverage so the disclaimer stays present in both the manual and generated templates

Why

Skills encode valuable domain knowledge, but applying them through an AI agent can be expensive and high-impact. The repository should say that clearly wherever skills are created or consumed.

Impact

Contributors and users now see the cost/scope warning in the skill artifact itself instead of having to infer it from context.

Validation

• npm test