fix: take the file edit limit in the panel from the agent's read cap

The limit was repeated in the page; a file of exactly the cap is read
in full, so it is editable too.

internal/proto/proto.go

@@ -91,6 +91,10 @@ const (
 	TaskSysUserAdd = "sys.useradd" // create a new OS user
 )
 
+// FSReadCap bounds how many bytes TaskFSRead returns (5 MiB). The panel lets a
+// file be edited only when it fits, so that a save never cuts it short.
+const FSReadCap = 5 * 1024 * 1024
+
 // TaskEnvelope is what the agent fetches for a task id.
 type TaskEnvelope struct {
 	Type string          `json:"type"`