feat: support provider-specific request options

Summary

Add a PROVIDER_OPTIONS configuration property that accepts a JSON object and forwards provider-specific options through AI SDK providerOptions.

Motivation

Custom OpenAI-compatible endpoints and model gateways often require request fields that aicommits does not expose as first-class configuration. The AI SDK already forwards provider-specific options to compatible providers, so this can be implemented without SDK changes.

Proposed behavior

• Accept JSON keyed by the configured AI SDK provider name, for example:
  bash复制

  aicommits config set 'PROVIDER_OPTIONS={"custom":{"customOption":"value"}}'

• Validate that the JSON root and each provider entry are objects.
• Merge configured options with built-in generation policy options. User values override matching keys while unrelated built-in values remain intact.
• Document the option and add configuration and provider-policy tests.

I plan to open a focused implementation PR for this proposal.