Fixes #13354

Summary

On Windows, every gh invocation that records telemetry produces a brief console window flash. The flash is tzutil.exe /g being spawned by the gh send-telemetry subprocess to resolve the local IANA timezone (via the transitive dependency github.com/thlib/go-timezone-local).

With Windows Terminal configured to keep windows open after exit (e.g. closeOnExit: never), these conhost instances accumulate.

Root cause

internal/telemetry/detach_windows.go spawns the telemetry subprocess with DETACHED_PROCESS. Per the Win32 process creation flags reference:

• DETACHED_PROCESS — child has no console at all. Any console-subsystem descendant (tzutil.exe) then allocates a fresh conhost on first stdio access, producing the visible flash.
• CREATE_NO_WINDOW — child runs as a console application without a visible console window. Descendants inherit the non-visible console and don't allocate a new one.

DETACHED_PROCESS and CREATE_NO_WINDOW are mutually exclusive. CREATE_NEW_PROCESS_GROUP remains compatible with the chosen flag.

Reproduction

Captured process tree on Windows 11:

gh.exe (parent shell invocation)
└── gh.exe send-telemetry    (spawned with DETACHED_PROCESS — no console)
    └── tzutil.exe /g        (allocates fresh conhost — visible flash)

gh send-telemetry invokes tzlocal.localTZfromTzutil() from github.com/thlib/go-timezone-local, which performs exec.Command("tzutil", "/g") with no Windows-specific window-suppression flags — so the inherited (absent) console determines visibility.

Fix

Replace DETACHED_PROCESS with CREATE_NO_WINDOW. Single-line behavioural change plus an explanatory comment.