store: link_set upserts instead of erroring on missing link

Creates the link if it doesn't exist, avoiding wasted agent turns
from the link_set/link_add confusion.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/hippocampus/store/ops.rs

@@ -359,7 +359,16 @@ impl Store {
             }
         }
         if !found {
-            return Err(format!("no link between {} and {}", source, target));
+            // Upsert: create the link if it doesn't exist
+            self.add_link(source, target, "link_set")?;
+            // Set the strength on the newly created link
+            for rel in self.relations.iter_mut().rev() {
+                if !rel.deleted && rel.source_key == source && rel.target_key == target {
+                    rel.strength = strength;
+                    break;
+                }
+            }
+            return Ok(0.0);
         }
         Ok(old)
     }