fix(session): remap compaction tail_start_id when forking (#24898)

Co-authored-by: spark4862 <spark4862@users.noreply.github.com>
Co-authored-by: Aiden Cline <aidenpcline@gmail.com>

packages/opencode/src/session/session.ts

@@ -616,12 +616,16 @@ export const layer: Layer.Layer<Service, never, Bus.Service | Storage.Service> =
         })
 
         for (const part of msg.parts) {
-          yield* updatePart({
+          const p: MessageV2.Part = {
             ...part,
             id: PartID.ascending(),
             messageID: cloned.id,
             sessionID: session.id,
-          })
+          }
+          if (p.type === "compaction" && p.tail_start_id) {
+            p.tail_start_id = idMap.get(p.tail_start_id)
+          }
+          yield* updatePart(p)
         }
       }
       return session