fix(opencode): clean up SSE heartbeat on server-initiated stream close

clearInterval(heartbeat) and unsub() only run inside stream.onAbort(),
which Hono only fires on client disconnect. When the server closes the
stream on InstanceDisposed via stream.close(), the interval and bus
subscription leak. Add cleanup before stream.close() in the
InstanceDisposed handler. Both calls are idempotent so the onAbort
fallback for normal client disconnects still works.

Author: s2bomb

packages/opencode/src/server/server.ts

@@ -515,6 +515,8 @@ export namespace Server {
                   data: JSON.stringify(event),
                 })
                 if (event.type === Bus.InstanceDisposed.type) {
+                  clearInterval(heartbeat)
+                  unsub()
                   stream.close()
                 }
               })