Expand CLI agent support for OpenCode, Codex, Gemini, and others

[Spherrrical]

We should expand planoai cli-agent to support additional developer CLIs like OpenCode, Codex, Gemini, and future tools through a single, consistent integration layer. Right now the flow is largely Claude-specific which highlights the need for a reusable launcher architecture instead of one-off implementations.

The goal is to let users route any supported CLI through Plano with minimal setup while reusing existing model_providers, aliases, and routing preferences. This work should include clean agent-specific env/argument mapping, consistent error handling, and a straightforward way to register new CLI agents without touching core control flow. It should also ship with brief docs and demo examples so each supported agent can be validated quickly end-to-end.

[Spherrrical]

Adding support for Codex at the moment, but hitting a blocker for v1/responses traffic that includes web-search tools. The conversion path from Response -> ChatCompletions only supports function tools. As a result, there is an immediate failure in routing.

Logs from Plano itself:

Failed to convert ResponsesAPIRequest to ChatCompletionsRequest: Unsupported conversion: WebSearchPreview tool is not supported in ChatCompletions API. Only function tools are supported.

This is using Claude Sonnet 4.6 inside of Codex. Using GPT 5.2 will pass through; but we should have it open to any model.