Add fx agent

[shindgew]

Summary

Adds fx (Vercel Labs) to the ACP registry.

fx is a native ACP server launched with fx acp. This entry pins the current stable 0.0.3 GitHub release archives for macOS and Linux (arm64 and x86_64).

Details

• Distribution: binary (./fx acp)
• Platforms: darwin-aarch64, darwin-x86_64, linux-aarch64, linux-x86_64
• Windows: not shipped by fx; validator will warn
• Repository: https://github.com/vercel-labs/fx
• Docs: https://fx.sh/docs/using-fx/acp
• License: Apache-2.0
• Icon: 16x16 monochrome SVG using currentColor

Auth note

fx currently expects fx login / fx setup (or AI_GATEWAY_API_KEY) before ACP starts:

• Without credentials, initialize returns an error instead of authMethods
• With credentials, initialize still returns "authMethods": []

That does not meet AUTHENTICATION.md, so verify-auth is expected to fail until fx advertises an agent or terminal auth method and can initialize without a pre-existing credential.

Validation

• Schema / entry structure validated
• Release archive URLs return HTTP 200
• Archive contains ./fx
• Live fx acp handshake on darwin-aarch64 confirms the auth behavior above