fix(process): disable detached spawn on Windows to fix empty exec output (#18035)

The supervisor's child adapter always spawned with `detached: true`,
which creates a new process group. On Windows Scheduled Tasks (headless,
no console), this prevents stdout/stderr pipes from properly connecting,
causing all exec tool output to silently disappear.

The old exec path (pre-supervisor refactor) never used `detached: true`.
The regression was introduced in cd44a0d01 (refactor process spawning).

Changes:
- child.ts: set `detached: false` on Windows, keep `detached: true` on
  POSIX (where it's needed to survive parent exit). Skip the no-detach
  fallback on Windows since it's already the default.
- child.test.ts: platform-aware assertions for detached behavior.

Fixes #18035
Fixes #17806

src/process/supervisor/adapters/child.test.ts

@@ -60,8 +60,15 @@ describe("createChildAdapter", () => {
       options?: { detached?: boolean };
       fallbacks?: Array<{ options?: { detached?: boolean } }>;
     };
-    expect(spawnArgs.options?.detached).toBe(true);
-    expect(spawnArgs.fallbacks?.[0]?.options?.detached).toBe(false);
+    // On Windows, detached defaults to false (headless Scheduled Task compat);
+    // on POSIX, detached is true with a no-detach fallback.
+    if (process.platform === "win32") {
+      expect(spawnArgs.options?.detached).toBe(false);
+      expect(spawnArgs.fallbacks).toEqual([]);
+    } else {
+      expect(spawnArgs.options?.detached).toBe(true);
+      expect(spawnArgs.fallbacks?.[0]?.options?.detached).toBe(false);
+    }
 
     adapter.kill();
 

src/process/supervisor/adapters/child.ts

@@ -42,11 +42,17 @@ export async function createChildAdapter(params: {
 
   const stdinMode = params.stdinMode ?? (params.input !== undefined ? "pipe-closed" : "inherit");
 
+  // On Windows, `detached: true` creates a new process group and can prevent
+  // stdout/stderr pipes from connecting when running under a Scheduled Task
+  // (headless, no console). Default to `detached: false` on Windows; on
+  // POSIX systems keep `detached: true` so the child survives parent exit.
+  const useDetached = process.platform !== "win32";
+
   const options: SpawnOptions = {
     cwd: params.cwd,
     env: params.env ? toStringEnv(params.env) : undefined,
     stdio: ["pipe", "pipe", "pipe"],
-    detached: true,
+    detached: useDetached,
     windowsHide: true,
     windowsVerbatimArguments: params.windowsVerbatimArguments,
   };
@@ -59,12 +65,14 @@ export async function createChildAdapter(params: {
   const spawned = await spawnWithFallback({
     argv: resolvedArgv,
     options,
-    fallbacks: [
-      {
-        label: "no-detach",
-        options: { detached: false },
-      },
-    ],
+    fallbacks: useDetached
+      ? [
+          {
+            label: "no-detach",
+            options: { detached: false },
+          },
+        ]
+      : [],
   });
 
   const child = spawned.child as ChildProcessWithoutNullStreams;