feat(opencode): add `privatemode-ai` provider

[msanft]

This adds provider configuration for Privatemode AI.

The default URL points to http://localhost:8080 as Privatemode AI relies on a locally-run proxy to provide end-to-end encryption.

Related PR in models.dev: https://github.com/anomalyco/models.dev/pull/602

Tested this locally with the relevant model entries in opencode.json.

Closes #7952