docs: add portable fetch skill

[ryanfowler]

Summary

• add a canonical portable fetch skill with concise agent guidance
• add progressive HTTP, diagnostics, gRPC, and WebSocket references
• add eval scenarios covering safe body parsing, TLS diagnostics, and gRPC discovery

Validation

• python3 -m json.tool skills/fetch/evals/evals.json
• git diff --cached --check

Note

• Prettier was unavailable in the local environment.