fix(copilot): add Windows platform guard to _fix_pipe_blocking_mode()

[Shazwazza]

Summary

_fix_pipe_blocking_mode() (introduced in #23) unconditionally imports cntl, a Unix-only module, causing ModuleNotFoundError: No module named 'fcntl' on Windows. O_NONBLOCK does not exist on Windows and pipes are always blocking, so the entire method is a no-op on that platform.

Fix

Add sys.platform == "win32" early return before the cntl import.

Validation

Cross-validated with GPT-5.4 (independent model review):

• sys.platform == "win32" is the right check — better targeted than os.name == "nt"; Cygwin/MSYS2/WSL correctly take the Unix path
• No-op on Windows is safe — subprocess.PIPE is blocking by default on Windows, so skipping won't reintroduce the BlockingIOError this method was designed to prevent
• Future improvement — os.set_blocking(fd, True) (available cross-platform in Python 3.12+) could replace the cntl approach entirely without platform branching, but that's a larger refactor beyond the scope of this bug fix

Tests

• est_skips_on_windows — verifies no-op on win32
• est_runs_on_unix — verifies the method proceeds past the guard on non-Windows

All 35 copilot provider tests pass.

[jrob5756]

I was about to merge then saw the lint issues. Can you take a look at those and we can merge this.