[Quality] Fix AI refinement — error handling, stub interfaces, provider deduplication

[ibourgeois]

Problems

1. No error handling: IdeaRefinementService::refine() makes an AI call with no try/catch. If Ollama is unavailable, an unhandled exception reaches the user as a 500.
2. Stub interfaces: IdeaRefinementAgent implements HasTools (returns []) and Conversational (returns []). Both add no value for a single-turn refinement — they should be removed.
3. Duplicate configuration: IdeaRefinementAgent has #[Provider('ollama')] and #[Model('llama3.1')] class attributes, but IdeaRefinementService overrides both at runtime. One source of truth should be chosen.

Required Changes

• IdeaRefinementService::refine(): wrap the AI call in try/catch; on failure, throw a user-friendly exception or return a safe fallback
• IdeaRefinementAgent: remove HasTools interface and its tools() method; remove Conversational interface and its messages() method
• IdeaRefinementAgent: remove #[Provider] and #[Model] class attributes — provider/model is already set in the service

Files

• app/Services/Ideas/IdeaRefinementService.php
• app/Ai/Agents/Ideas/IdeaRefinementAgent.php