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.

Parallel PR in models.dev: anomalyco/models.dev#602

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Search Results

No duplicate PRs found.

The search results only return the PR itself (PR #6931 - "Add privatemode-ai provider"), which indicates this is a new addition without existing duplicate or related open PRs in the repository.

All other results are related to different provider additions or features (Amazon Bedrock, Cursor, Ollama Cloud, GitHub Action custom providers, etc.) but are not duplicates of this specific Privatemode AI provider PR.