Add HTTP version aliases and clearer protocol errors

[ryanfowler]

Summary

• Add hidden --http1, --http2, and --http3 aliases alongside --http
• Route HTTP version selection through shared CLI helpers so config, HTTP, TLS, WebSocket, and inspection paths agree
• Improve the plain-HTTP/2 error to explain the h2c limitation
• Update docs and completion metadata to match the new aliases

Testing

• Unit tests added for alias parsing and updated HTTP version validation
• Integration test coverage updated to verify --http2 fails with the new guidance