basic undo feature (#1268)

Co-authored-by: adamdotdevin <2363879+adamdottv@users.noreply.github.com>
Co-authored-by: Jay V <air@live.ca>
Co-authored-by: Aiden Cline <63023139+rekram1-node@users.noreply.github.com>
Co-authored-by: Andrew Joslin <andrew@ajoslin.com>
Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Tobias Walle <9933601+tobias-walle@users.noreply.github.com>

packages/opencode/src/session/message-v2.ts

@@ -272,6 +272,13 @@ export namespace MessageV2 {
         part: Part,
       }),
     ),
+    PartRemoved: Bus.event(
+      "message.part.removed",
+      z.object({
+        messageID: z.string(),
+        partID: z.string(),
+      }),
+    ),
   }
 
   export function fromV1(v1: Message.Info) {