story-building

Use when user says "build stories", "generate stories", "create stories", "brainstorm stories", "brainstorm features", "create stories for [node]", or asks for new story ideas. Complete story lifecycle - generates evidence-based stories from git commits and gap analysis, validates quality, vets for conflicts, and retries up to 10 times if duplicates detected. Polishes stories with hold_reason='polish' FIRST before generating new ones. Supports multi-node batching. Does NOT commit - leaves that to caller.