Add toolnexus to Artificial Intelligence

[deemwario]

toolnexus — agent SDK with MCP tools and agent skills
built in (tool-calling loop, skills injection, six unified tool sources, conversation memory).
Byte-identical output across seven ports (JS, Python, Go, Java, C#, Elixir, Clojure) verified by
one shared conformance suite; the Elixir port is hex-installable
(https://hex.pm/packages/toolnexus), documented (https://muthuishere.github.io/toolnexus/), and
tested (CI badge in the README).

Placed alphabetically next to Jido and AshAI, the closest existing entries in scope.

[deemwario]

Marking this draft for now — found a CI issue on the linked project after opening this (tests requiring a live API key run unconditionally instead of skipping when absent), so the badge is red right now. Will mark ready once it's fixed; apologies for the noise in the meantime.

[deemwario]

CI is green now (verified: https://github.com/muthuishere/toolnexus/actions — latest run passing). Marking ready for review.