feat: NodePromoter — daily anchor emergence (Neural Mesh RAG Phase 4) #2119

Context

Part of Neural Mesh RAG (#1994), Phase 4, Task 4.3.

Implementation

• Find hot non-anchor nodes (access_count>50, min 5 edges)
• Generate neighborhood summary via LLM
• Embed summary, store in ChromaDB mesh_anchors collection
• Promote node in PostgreSQL (is_anchor=True)
• Demote stale anchors (access dropped, not accessed 30d)

Files

• Create: autobot-backend/services/mesh_brain/node_promoter.py
• Test: autobot-backend/services/mesh_brain/node_promoter_test.py