[FEATURE]: Document LLMTR provider in providers.mdx

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• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

providers.mdx is missing a section for LLMTR, even though opencode already works with it.

LLMTR is an OpenAI-compatible gateway. It was merged into models.dev and is live in the catalog at https://models.opencode.ai/api.json ("llmtr", npm @ai-sdk/openai-compatible, env LLMTR_API_KEY), so the provider resolves through the generic openai-compatible path with no runtime code in opencode.

Verified today on the released opencode-ai@1.18.18, with LLMTR_API_KEY set:

$ opencode models | grep llmtr
llmtr/gemma-4
llmtr/magibu-11b-v8
llmtr/medgemma-4b
llmtr/qwen3-6-35b
llmtr/sincap
llmtr/trendyol-7b

$ opencode run --model llmtr/gemma-4 "Reply with exactly: OK"
> build · gemma-4
OK

So the connect/models flow works but there is nothing in the docs telling anyone it exists. Every other provider in the directory has a short section; LLMTR should have one too.

This was previously #31315 with PR #31316. Both were closed by the automated cleanup/stale bots rather than on merit, and neither can be reopened, so I am refiling per the stale bot's instruction to open a new issue.

I have the docs change ready and will open a PR against this issue.