feat: add mlx-agent production specialists - #186

What changed

• Add an mlx-agent API-client provider for production specialists. It sends Chat Completions requests to an mlx-agent gateway's /v1/chat/completions with the model default_model, which the gateway resolves to the model it serves.
• Each request sets chat_template_kwargs.enable_thinking to false and sends the turn's response schema as a json_schema response_format.
• The endpoint must be a loopback origin. allowRemote, model and credentials are optional for this provider; apiKeyEnv sends a bearer token when set.
• Document the provider in the production specialists guide, the configuration reference, the README and the changelog.

Why

• Specialists had no way to use a local model served by mlx-agent: the Claude and Codex providers are fixed to official endpoints, the other providers target agent gateways, and the Ollama adapter speaks Ollama's API.

Verification

• npm test
• node --test test/producer-api-client.test.mjs test/producer-providers.test.mjs test/producer-service.test.mjs: 135 pass. The two new tests fail on main with provider must be equal to one of the allowed values.
• Against a local mlx-agent gateway, a creative inspect task returned a schema-valid proposal and the coordinator ran its reads. Its fourth read targeted an empty clip slot and ended the task.
• npm run docs:test
• npm run verify:package, if packaging, the CLI, or the bridge install changed
• npm run docs:tools, if a tool contract changed
• Checked against a running Live, if Live-side behavior changed (Live version: )

Contract impact

• No tool, resource, or prompt contract change
• Tool contract changed and reference/tools.md regenerated