feat(wizard): add CommonStack as a unified API gateway provider

[flhoildy]

Summary

Adds CommonStack as a selectable LLM provider in
assets/configure_mykey.py. CommonStack is an OAI/Anthropic-compatible
unified gateway that aggregates flagship models from OpenAI, Anthropic,
Google, DeepSeek, MiniMax, Zhipu, xAI, Moonshot, Qwen and Xiaomi behind
a single key — same shape as the existing openrouter entry.

Details

• Single-entry addition to LLM_PROVIDERS, between openrouter and crs.
• type: native_oai, apibase: https://api.commonstack.ai/v1,
  api_mode: chat_completions. Standard Bearer auth — no quirks.
• model_choices kept to 2 flagship ids mirroring openrouter's shape,
  since probe_models() fetches the live /v1/models list at config time
  (returns ~55 ids currently). model_choices is only consulted by
  _fallback_model() on probe failure.
• Default model is anthropic/claude-opus-4-7, matching the project's
  pattern of defaulting to the flagship Claude.

Test plan

• from configure_mykey import LLM_PROVIDERS imports cleanly.
• Provider menu order: cc_relay → openrouter → commonstack → crs → gmi.
• probe_models() against live /v1/models returns 55 ids.
• End-to-end configure_llm(commonstack) with mocked stdin confirms
  the live list (alphabetical, starts with anthropic/claude-haiku-4-5)
  populates the picker, not the 2-item fallback.

[shenhao-stu]

Thanks for the PR! I don't think we'll merge this one though.

CommonStack has the same shape as our existing cc_relay / openrouter / crs native_oai channels, and the repo already ships a generic OpenAI / Responses adapter (see memory/llm-api-adapter/ and the dual-mode images_api + responses_tool fallback in image_gen_api.py). Any OAI-compatible gateway can be wired up by adding a single provider entry to mykey.py — no need to bake every gateway into the wizard's built-in list.

Going to close this for now. Thanks again!