[kyuminjo]

Adds PopNow as a built-in OpenAI-compatible provider, following the "Compatible providers" path in the contributing guide.

PopNow serves chat models from several vendors behind one API key. Its model ids keep a vendor prefix and a task suffix, for example deepseek/v4-flash/chat, so popnow/deepseek/v4-flash/chat relies on the existing first-slash split.

• pkg/model/provider/catalog/providers.json: catalog entry, with POPNOW_API_KEY and a low-cost default model
• docs/providers/popnow/, nav, overview and model reference: docs
• examples/popnow.yaml: runnable example
• tests: auto-selection, default model, alias behaviour, system message merging, and the models.dev absence list

Verification: live requests to https://api.popnow.ai/v1/chat/completions with deepseek/v4-flash/chat returned a normal completion and a valid tool call. I did not run go test locally, so the test changes are relying on CI here.

Disclosure: I work on PopNow.