types/process: expose writable stdin state flags

src/agents/bash-process-registry.ts

@@ -23,6 +23,9 @@ export type SessionStdin = {
   // When backed by a real Node stream (child.stdin), this exists; for PTY wrappers it may not.
   destroy?: () => void;
   destroyed?: boolean;
+  writable?: boolean;
+  writableEnded?: boolean;
+  writableFinished?: boolean;
 };
 
 export interface ProcessSession {