Add a Nuxt static-output compatibility fixture

[Ryan-yang125]

Goal: add a frozen Nuxt fixture and regression coverage for generated static output.

Acceptance criteria:

• The fixture includes .output/public/index.html and a representative server-output directory.
• The detector selects .output/public and rejects server output.
• The case runs inside the existing Skill check in CI.
• The test performs no network requests or deployments.