feat: support authenticated Codex and Claude CLI providers

Feature request

Add first-class codex and claude providers that invoke a locally installed, authenticated Codex CLI or Claude Code CLI. This lets users generate commit messages using an existing CLI subscription without configuring a separate API key.

Why?

The current hosted providers require separate API credentials and usage billing. Users who already subscribe to Codex or Claude Code expect a local CLI integration to reuse that authenticated session, with the same simple aicommits setup workflow.

Alternatives

• Use the existing OpenAI or Anthropic APIs. This requires a separate API key and billing setup.
• Run a local model through Ollama or LM Studio. This adds local model installation and hardware requirements.
• Use a standalone wrapper around codex exec or claude --print. This duplicates aicommits' staging, formatting, prompt, hook, and confirmation workflows.

Additional context

A proposed implementation is ready. It adds both providers to setup, invokes each CLI in non-interactive mode, disables Claude tools, runs Codex in a read-only sandbox, passes only the staged diff on stdin, and supports persistent default prompts with command-line overrides.