agents: add DEMOTE action for redundancy cleanup

New action type that halves a node's weight (min 0.05), enabling
extractors to mark redundant nodes for decay without deleting them.

Parser, apply logic, depth computation, and display all updated.

poc-memory/src/agents/consolidate.rs

@@ -231,6 +231,8 @@ pub fn apply_consolidation(store: &mut Store, do_apply: bool, report_key: Option
                     println!("  WRITE {}", key),
                 knowledge::ActionKind::Refine { key, .. } =>
                     println!("  REFINE {}", key),
+                knowledge::ActionKind::Demote { key } =>
+                    println!("  DEMOTE {}", key),
             }
         }
         println!("\nTo apply: poc-memory apply-consolidation --apply");