[Proposal]: Add explicit OpenAI-compatible and Responses text providers

Summary

Add an explicit custom text-provider contract while preserving AI Gateway as the default:

• --provider openai-compatible for Chat Completions
• --provider openai-responses for Responses streaming
• --base-url, --api-key-env, optional --models-url, and existing --model
• equivalent AI_CLI_* environment variables
• fixed, non-secret attribution headers from PAPERCLIP_AGENT_ID / PAPERCLIP_RUN_ID
• fail closed for image, video, speech, and transcription in custom modes
• redact configured keys, bearer auth, and URL userinfo from errors

The existing package metadata declares Apache-2.0, but the repository and npm tarball omit the license text. The proposal also adds the canonical Apache-2.0 LICENSE to the repository and packed CLI.

Motivation

Self-hosted routers and local model gateways need a supported text path that cannot silently fall back to Vercel AI Gateway. API-key environment indirection keeps keys out of command history, and custom model listing should use the selected endpoint rather than contacting Gateway.

Implementation notes

A reviewed reference implementation is available in valkyriweb/ai-cli PR 1: https://github.com/valkyriweb/ai-cli/pull/1

Exact reviewed head: 7803a8f7a53fa37d113adb0b469c76c988166165

The provider selection is centralized in src/lib/provider.ts; model discovery dispatches by provider; Responses mode uses streaming because some compatible routers require stream: true. Gateway code paths and defaults are unchanged.

The bundled ai-cli skill is extended rather than adding a competing skill, with a focused Agent Skill authoring reference.

Testing notes

• deterministic fetch tests assert exact /models, /chat/completions, and /responses URLs
• assert allowlisted headers and no ai-gateway.vercel.sh request in custom modes
• unsupported modality and redaction tests
• full typecheck, format, lint, tests, and build
• real synthetic Responses smoke through a self-hosted router using a cheap text model
• independent exact-head review and TruffleHog scan are clean

Risks or tradeoffs

Custom modes intentionally support text only. OpenAI-compatible model metadata is less rich than Gateway metadata, so pricing/provider endpoint details are unavailable. Responses mode streams internally and collects the final text to preserve current CLI output semantics.

Co-contributor credit

Yes, credit @valkyriweb.

Checklist

• I searched existing issues for duplicates.
• I understand pull requests are limited to collaborators and maintainers will handle implementation.