fix(files): a workspace whose folder was deleted leaves the Files places

The lane payload carries folder_missing (ENOENT/ENOTDIR only). Files hides
such a workspace and never reopens it as the default place; the Workspace
sidebar strikes its name through with a hint to remove it from its menu.
A folder that cannot be read no longer keeps the previous folder's entries
under its error, and a change in Files re-reads the workspace list.

client/src/lib/types.ts

@@ -2731,6 +2731,8 @@ export interface Lane {
   stage_seconds: number | null;
   last_event_seconds: number | null;
   liveness: "active" | "idle" | "dead";
+  /** The workspace's folder no longer exists (deleted from Files or a shell). */
+  folder_missing?: boolean;
   /** Stage id the server's heuristic infers from ingested tool events, or null
    *  if no signal was seen. Independent of `stage` (the agent's own declaration). */
   detected_stage: string | null;