cli(windows): pass sys.stdio handles explicitly to powershell.exe

The previous Write-Host capture attempts (47432b0b -Command + *>&1
and f2c2b3f3 [Console]::Out mirror in setup.ps1) still produced an
empty update.log on windows-latest because the powershell.exe child
had no stdio handles at all to write to.

Root cause: subprocess.run on Windows with the default close_fds=True
(Python 3.7+ default) sets bInheritHandles=False on CreateProcess.
Combined with CREATE_NO_WINDOW (added by _windows_hidden_subprocess_
kwargs in non-TTY runs), the child gets:
  - no console (CREATE_NO_WINDOW)
  - no inherited std handles (bInheritHandles=False)
GetStdHandle in the child returns INVALID_HANDLE_VALUE, so even
[Console]::Out.WriteLine and Write-Output -- not just Write-Host --
write into the void.

Fix: pass stdout=sys.stdout, stderr=sys.stderr (and stdin) when
running the setup script on Windows. With explicit handles, Python's
subprocess sets up PROC_THREAD_ATTRIBUTE_HANDLE_LIST containing the
std handles + bInheritHandles=True, so the child inherits exactly
the three std handles regardless of close_fds=True. CREATE_NO_WINDOW
still applies (no transient console window), but the child can now
write to the inherited stdout file handle, which lands on bash's
`tee logs/update.log` in CI.

A small _stream_for_subprocess helper guards against test harnesses
that swap sys.stdout for a stream without a real fileno (pytest
capsys, in-memory IO buffers, etc) -- those fall back to None so
subprocess uses its default.

Verified locally on PowerShell 7.4.6 / Linux that the explicit
stdout handoff doesn't regress the existing direct-inherit path,
and the marker line "prebuilt up to date and validated" reaches
both the child's stdout and a parent `tee` consumer.

unsloth_cli/commands/studio.py

@@ -133,6 +133,26 @@ def _windows_hidden_subprocess_kwargs() -> dict[str, object]:
     return kwargs
 
 
+def _stream_for_subprocess(stream):
+    """Return *stream* if it has a real OS file descriptor, else None.
+
+    subprocess.run on Windows refuses to inherit std handles unless
+    they're passed explicitly (otherwise close_fds=True forces
+    bInheritHandles=False, and a CREATE_NO_WINDOW child ends up with
+    no stdio at all). When sys.stdout / sys.stderr is a real fd-backed
+    stream we want to hand it through; when it's been captured by a
+    test harness (pytest's capsys, an in-memory wrapper, etc) we fall
+    back to None so subprocess uses its default.
+    """
+    if stream is None:
+        return None
+    try:
+        stream.fileno()
+    except (AttributeError, OSError, ValueError):
+        return None
+    return stream
+
+
 def _studio_venv_python() -> Optional[Path]:
     """Return the studio venv Python binary, or None if not set up."""
     if platform.system() == "Windows":
@@ -1015,9 +1035,26 @@ def _run_setup_script(*, verbose: bool = False) -> None:
                 f"& '{script_pwsh_literal}' *>&1",
             ]
         )
+        # Explicitly hand stdin/stdout/stderr to the child so the
+        # CI tee actually sees setup.ps1's output. Without this,
+        # subprocess.run on Windows uses close_fds=True (default,
+        # since Python 3.7) which sets bInheritHandles=False on
+        # CreateProcess. With CREATE_NO_WINDOW also set (via
+        # _windows_hidden_subprocess_kwargs in non-TTY runs), the
+        # child has neither a console nor any inherited std
+        # handles, so PowerShell's Write-Host -- and even
+        # [Console]::Out.WriteLine -- writes to nothing. Passing
+        # stdout=sys.stdout / stderr=sys.stderr makes Python set up
+        # PROC_THREAD_ATTRIBUTE_HANDLE_LIST with the std handles
+        # explicitly inheritable, which works alongside
+        # CREATE_NO_WINDOW. Empty update.log on the windows-latest
+        # CI was the smoking gun (run 25533694490 and 25534292239).
         result = subprocess.run(
             powershell_args,
             env = env,
+            stdin = _stream_for_subprocess(sys.stdin),
+            stdout = _stream_for_subprocess(sys.stdout),
+            stderr = _stream_for_subprocess(sys.stderr),
             **_windows_hidden_subprocess_kwargs(),
         )
     else: