fix: prevent FD leaks in child process cleanup

- Destroy stdio streams (stdin/stdout/stderr) after process exit
- Remove event listeners to prevent memory leaks
- Clean up child process reference in moveToFinished()
- Also fixes model override handling in agent.ts

Fixes EBADF errors caused by accumulating file descriptors
from sub-agent spawns.

src/agents/bash-process-registry.ts

@@ -157,6 +157,38 @@ export function markBackgrounded(session: ProcessSession) {
 
 function moveToFinished(session: ProcessSession, status: ProcessStatus) {
   runningSessions.delete(session.id);
+
+  // Clean up child process stdio streams to prevent FD leaks
+  if (session.child) {
+    // Destroy stdio streams to release file descriptors
+    session.child.stdin?.destroy?.();
+    session.child.stdout?.destroy?.();
+    session.child.stderr?.destroy?.();
+
+    // Remove all event listeners to prevent memory leaks
+    session.child.removeAllListeners();
+
+    // Clear the reference
+    delete session.child;
+  }
+
+  // Clean up stdin wrapper - call destroy if available, otherwise just remove reference
+  if (session.stdin) {
+    // Try to call destroy/end method if exists
+    if (typeof session.stdin.destroy === "function") {
+      session.stdin.destroy();
+    } else if (typeof session.stdin.end === "function") {
+      session.stdin.end();
+    }
+    // Only set flag if writable
+    try {
+      (session.stdin as { destroyed?: boolean }).destroyed = true;
+    } catch {
+      // Ignore if read-only
+    }
+    delete session.stdin;
+  }
+
   if (!session.backgrounded) {
     return;
   }