Add API versioning skill

[PAMulligan]

Description

Create a dedicated skill for implementing API versioning in Nerva-generated APIs. While the pipeline config defines versioning defaults, there is no skill that generates the versioning infrastructure.

Why

API versioning is complex in practice — it touches routing, middleware, documentation, deprecation headers, and client communication. A dedicated skill provides the full implementation, not just configuration.

Acceptance Criteria

• Create `.claude/skills/api-versioning/SKILL.md`
• Implement URL prefix versioning (`/v1/`, `/v2/`):
  • Version-scoped route groups
  • Version extraction middleware
  • Default version fallback
• Implement header-based versioning (optional):
  • `Accept-Version` header parsing
  • Content negotiation
• Generate deprecation infrastructure:
  • `Sunset` header middleware
  • `Deprecation` header on old versions
  • Version sunset schedule configuration
• Generate version migration utilities:
  • Request/response transformers between versions
  • Schema evolution patterns
• Add integration tests for multi-version APIs
• CI passes

[github-actions]

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[github-actions]

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