Implement AI-powered summary generation for devlog summary create --ai

[gustmrg]

Implement the AI summary generation flow documented in the README.

Context:

• README.md documents OpenRouter-backed summary rewriting
• Current code accepts --ai but does not call any LLM provider
• The saved summary is always built locally

Expected behavior:

• When --ai is enabled, collect the relevant entries and send them to the configured LLM
• Generate a polished summary based on the requested style
• Save the AI-generated summary alongside metadata indicating style and AI usage
• Return a clear error if AI is enabled but required configuration is missing

Implementation notes:

• Read provider/model/api key configuration from the existing config structure
• Handle missing API key and provider failures gracefully
• Keep non-AI generation available as a fallback path

Acceptance criteria:

• devlog summary create --ai performs an actual LLM request
• The configured model/provider are used
• Failures are reported clearly
• The resulting summary content differs from the basic local formatter when AI is enabled