Add subscription-backed Codex and Claude CLI providers

[tonibergholm]

Summary

Add optional Codex and Claude Code CLI providers so meat can use an existing ChatGPT/Codex or Claude subscription login without requiring separate API credentials.

meat -provider codex
meat -provider claude

The existing OpenAI and Anthropic API-key providers remain unchanged and continue to be selected by the default auto provider.

Implementation

• Add -provider and MEAT_PROVIDER.
• Support auto, openai, anthropic, codex, and claude.
• Invoke the official Codex and Claude Code CLIs in non-interactive structured-output mode.
• Translate CLI responses into meat’s existing provider-neutral text and tool_use blocks.
• Keep edit-plan validation and tool execution inside meat.
• Separate cache entries by provider and model.
• Support optional MEAT_CODEX_CLI and MEAT_CLAUDE_CLI command overrides.
• Document installation, authentication, and usage.

Authentication and isolation

The adapters do not extract or reuse OAuth tokens. Authentication remains entirely owned by the official provider CLIs.

Subscription-backed turns are ephemeral:

• Claude Code runs with built-in tools and customizations disabled.
• Codex runs read-only in an empty temporary workspace.
• Codex user configuration, rules, and inherited shell environment are disabled.
• Repository reads still go through meat’s existing confined tool protocol.

Trade-offs

CLI-backed calls have more startup and agent-context overhead than direct API requests and count against the user’s applicable Codex or Claude subscription limits. They are opt-in and do not affect existing behavior.

Validation

• go test ./...
• go vet ./...
• git diff --check
• Live authenticated smoke test using meat -provider codex.
• Live authenticated smoke test using meat -provider claude.