feat(examples): add Claude API model-retirement repair example

[GBX-Max1220]

Summary

Adds a reproducible SkillHone example for anthropics/skills issue anthropics/skills#1603, which reported that four unavailable Claude model IDs remained listed as Legacy or Deprecated while Opus 4.5 and Sonnet 4.5 were still active.

The example fetches the public upstream Skill at a pinned commit, verifies its commit and blob IDs, and generates a fixture with a focused .test contract. It vendors no upstream source and requires no model credentials or private data.

Validation

• Confirmed the generated baseline test fails on the reported retired-model classification.
• Confirmed the test passes after a scoped repair in a temporary fixture copy, while preserving the two reported active models.
• pnpm test: 31/31 passed in WSL.
• Both SkillHone static_check.py checks passed.
• gitleaks dir and gitleaks git: no leaks found.

The fixture captures the historical report; it is not a live model-catalog validator.