fix: prevent Gemini orphan processes when switching modes

Kill entire process tree instead of just the direct child to prevent
orphans when Gemini CLI ignores SIGTERM. Implements killProcessTree that
collects all PIDs synchronously, signals children first, then waits for
termination with auto-escalation to SIGKILL after 2 seconds. Also removes
Node.js signal option from spawn since built-in abort handling doesn't
handle grandchildren.

cli/src/utils/process.ts

@@ -43,12 +43,91 @@ export async function killProcess(pid: number, force: boolean = false): Promise<
 
   try {
     process.kill(pid, force ? 'SIGKILL' : 'SIGTERM');
+    await waitForProcessToDie(pid, force);
     return true;
   } catch {
     return false;
   }
 }
 
+/**
+ * Recursively collects all descendant PIDs of a process (depth-first).
+ * Returns PIDs in child-first order (leaves first, root last).
+ */
+function collectProcessTree(pid: number): number[] {
+  const pids: number[] = [];
+
+  try {
+    const result = spawn.sync('pgrep', ['-P', pid.toString()], { encoding: 'utf8' });
+    if (result.stdout) {
+      const childPids = result.stdout.trim().split('\n').filter(Boolean).map(Number);
+      for (const childPid of childPids) {
+        pids.push(...collectProcessTree(childPid));
+      }
+    }
+  } catch {
+    // pgrep may not be available
+  }
+
+  pids.push(pid);
+  return pids;
+}
+
+/**
+ * Kills a process and all its descendants.
+ * Signals are sent synchronously (children first) to work in exit handlers,
+ * then waits asynchronously for processes to die.
+ */
+async function killProcessTree(pid: number, force: boolean): Promise<boolean> {
+  // Collect all PIDs first (sync) - returns in child-first order
+  const pids = collectProcessTree(pid);
+
+  // Signal all processes synchronously (children first, then root)
+  const signal = force ? 'SIGKILL' : 'SIGTERM';
+  for (const p of pids) {
+    try {
+      process.kill(p, signal);
+    } catch {
+      // Process may have already exited
+    }
+  }
+
+  // Wait for processes to die (async) - wait for root last
+  for (const p of pids) {
+    await waitForProcessToDie(p, force);
+  }
+
+  return true;
+}
+
+/**
+ * Waits for a process to die, escalating to SIGKILL if SIGTERM doesn't work.
+ */
+async function waitForProcessToDie(pid: number, force: boolean): Promise<void> {
+  const maxWait = 2000;
+  const pollInterval = 20;
+  let waited = 0;
+
+  while (isProcessAlive(pid) && waited < maxWait) {
+    await new Promise(r => setTimeout(r, pollInterval));
+    waited += pollInterval;
+  }
+
+  // If SIGTERM didn't work and we haven't tried SIGKILL yet, escalate
+  if (!force && isProcessAlive(pid)) {
+    try {
+      process.kill(pid, 'SIGKILL');
+    } catch {
+      return;
+    }
+    waited = 0;
+    while (isProcessAlive(pid) && waited < 1000) {
+      await new Promise(r => setTimeout(r, pollInterval));
+      waited += pollInterval;
+    }
+  }
+}
+
 export async function killProcessByChildProcess(
   child: ChildProcess,
   force: boolean = false
@@ -59,13 +138,10 @@ export async function killProcessByChildProcess(
   }
 
   if (isWindows()) {
+    // Windows taskkill /T already kills the entire process tree
     return killProcess(pid, force);
   }
 
-  try {
-    child.kill(force ? 'SIGKILL' : 'SIGTERM');
-    return true;
-  } catch {
-    return false;
-  }
+  // Kill entire process tree on Unix to prevent orphan processes
+  return killProcessTree(pid, force);
 }

cli/src/utils/spawnWithAbort.ts

@@ -43,9 +43,12 @@ export async function spawnWithAbort(options: SpawnWithAbortOptions): Promise<vo
     };
 
     await new Promise<void>((resolve, reject) => {
+        // Note: We intentionally do NOT pass signal to spawn() because Node.js's
+        // built-in abort handling only kills the direct child, not grandchildren.
+        // Instead, we handle abort ourselves using killProcessByChildProcess which
+        // kills the entire process tree to prevent orphan processes.
         const child = spawn(options.command, options.args, {
             stdio,
-            signal: options.signal,
             cwd: options.cwd,
             env: options.env,
             shell: options.shell
@@ -57,6 +60,14 @@ export async function spawnWithAbort(options: SpawnWithAbortOptions): Promise<vo
             if (abortKillTimeout) {
                 return;
             }
+            // First, try graceful termination of entire process tree
+            if (child.exitCode === null && !child.killed) {
+                logDebug(`Abort signal received, killing process tree (pid=${child.pid}) with SIGTERM`);
+                // Note: We don't await here because we're in a sync callback,
+                // but killProcessByChildProcess now waits for processes to die internally
+                void killProcessByChildProcess(child, false);
+            }
+            // Set timeout for forceful kill if graceful doesn't work
             abortKillTimeout = setTimeout(() => {
                 if (child.exitCode === null && !child.killed) {
                     logDebug('Abort timeout reached, sending SIGKILL');