fix(respawn): use screen -X stuff for reliable input delivery

When writing to an attached PTY connected to screen, the input doesn't
reliably reach Claude CLI. This fix uses `screen -X stuff` command to
send input directly to the screen session, bypassing the PTY attachment.

Research findings:
- Claude CLI uses Ink (React for CLI) for terminal UI
- Ink's parseKeypress detects \r as key.return (Enter)
- screen -X stuff with $'...' syntax reliably sends literal characters

Changes:
- Add ScreenManager.sendInput() using screen -X stuff
- Modify Session.write() to prefer screen -X stuff when available
- Keep \r as Enter key (what Ink expects for key.return)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/session.ts

@@ -898,7 +898,18 @@ export class Session extends EventEmitter {
   }
 
   // Send input to the PTY (for interactive sessions)
+  // When using screen, prefers screen -X stuff for reliable input delivery
   write(data: string): void {
+    // Try screen -X stuff first if we have a screen session
+    // This is more reliable for sending Enter key to Claude CLI
+    if (this._screenManager && this._screenSession) {
+      const success = this._screenManager.sendInput(this.id, data);
+      if (success) {
+        return;
+      }
+      // Fall through to PTY write if screen command failed
+    }
+
     if (this.ptyProcess) {
       this.ptyProcess.write(data);
     }