agent: remove memory_delete from tools, supersede transfers links

- memory_delete no longer exposed to agents - use supersede instead
- memory_supersede now transfers all edges from old node to new node
  (keeps whichever strength is higher if new node already has the link)
  This preserves graph structure during consolidation.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/memory.rs

@@ -208,7 +208,7 @@ memory_tool!(graph_trace, ref, key: [str]);
 
 // ── Definitions ────────────────────────────────────────────────
 
-pub fn memory_tools() -> [super::Tool; 19] {
+pub fn memory_tools() -> [super::Tool; 18] {
     use super::Tool;
     macro_rules! tool {
         ($name:ident, $desc:expr, $params:expr) => {
@@ -263,11 +263,7 @@ pub fn memory_tools() -> [super::Tool; 19] {
             "properties": { "source": {"type": "string"}, "target": {"type": "string"} },
             "required": ["source", "target"]
         }"#),
-        tool!(memory_delete, "Delete a memory node.", r#"{
-            "type": "object",
-            "properties": { "key": {"type": "string"} },
-            "required": ["key"]
-        }"#),
+        // NOTE: memory_delete not exposed to agents - use memory_supersede instead
         tool!(memory_history, "Show version history for a node.", r#"{
             "type": "object",
             "properties": { "key": {"type": "string"}, "full": {"type": "boolean"} },