[Feature]: docs example for PZERO on OpenAI-API-compatible

### Self Checks

- [x] I have read the Contributing Guide and Language Policy.
- [x] I have searched for existing issues, including closed ones. Search for `pzero` returned none.
- [x] I confirm that I am using English to submit this report.
- [x] I have not modified this template and filled in the required fields.

### 1. Is this request related to a challenge you're experiencing? Tell me about your story.

I was trying to point a Dify agent at PZERO prepaid inference using the existing OpenAI-API-compatible provider. The plugin already does this. What I could not find is a named example with the real host, key format, and model id, so I had to guess whether the API endpoint URL should be `https://api.pzero.studio/v1` or the full chat path.

A docs example (or a marketplace plugin README row like the TokenLab one) would let Dify users paste and run. No first-party PZERO plugin required.

Settings that work on the existing OpenAI-API-compatible provider:

- Model type: LLM
- Model name: `deepseek-v4-flash` (or any live text id from `GET https://api.pzero.studio/v1/models`)
- API Key: `pzero_…` from https://pzero.studio/agents
- API endpoint URL: `https://api.pzero.studio/v1` (the `/v1` root, not `/v1/chat/completions`)
- Completion mode / chat type: Chat

Get a key: https://pzero.studio/agents. Sign-in is free. Usage is paid. Minimum top-up is 1 USDC on Base. There are no starter credits.

Smoke against the same host Dify will call:

```bash
curl -sS -X POST "https://api.pzero.studio/v1/chat/completions" \
-H "Authorization: Bearer $PZERO_API_KEY" \
-H "Content-Type: application/json" \
-d '{"model":"deepseek-v4-flash","messages":[{"role":"user","content":"Hello from PZERO"}],"stream":false}'
```

Public catalog, no key: `GET https://api.pzero.studio/v1/models`.

I am not asking for a bundled first-party provider next to OpenAI. The OpenAI-API-compatible plugin is the existing seam.

Every AI Credit is at least 20% off $1 list. Usage is paid. Do not start implementation on my behalf until the board says Ready; the settings are for users today.

### 2. Additional context or comments

PZERO also serves `POST /v1/responses`. For this Dify plugin, use chat completions.

Plugin: https://marketplace.dify.ai/plugin/langgenius/openai_api_compatible

### 3. Can you help us with this feature?

I can supply copy and the live curl above. I cannot open a PR from this account (no public fork).

Start with the existing OpenAI-API-compatible provider documentation and its marketplace entry, using the TokenLab README row as the cited example. Add a PZERO example with the supplied endpoint, key format, model lookup, chat-completions settings, and curl smoke test; done means a Dify user can paste the settings and verify a response.

Written by the indexing model from the issue text.