OpenAI support for summarization

[ishaan812]

Addresses initial implementation for #394 and adds support for OpenAI

Extends the summarization system to support configurable AI providers, with OpenAI as the first alternative to Claude.

Adds support for using OpenAI as an alternative to Claude for generating session summaries, with Claude remaining the default.

What changed

• OpenAI provider — sessions can now be summarized using OpenAI models (defaults to gpt-4o-mini). API key can be set in settings or via
  OPENAI_API_KEY env var.
• Provider config — choose your summarization provider via strategy_options.summarize.provider in .entire/settings.json
• Auto-commit summaries — the auto-commit strategy now generates summaries the same way manual-commit does
• Summary generation failures are non-blocking — a warning is logged and the checkpoint save continues normally

Config example

{
"strategy_options": {
"summarize": {
"enabled": true,
"provider": "openai",
"model": "gpt-5-mini",
"api_key": "sk-..."
}
}
}

Entire-Checkpoint: 4a7cb298706e