T1.2: Create src/platform/winproc; move configureHiddenWindow

Extract the hidden-window logic out of src/core runner and autostart files
into a new platform/winproc package with per-OS build-tag files. All call
sites updated to use winproc.ConfigureHiddenWindow. Builds clean on both
Windows and Linux; all tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/platform/winproc/winproc_other.go

@@ -0,0 +1,10 @@
+//go:build !windows
+
+package winproc
+
+import "os/exec"
+
+// ConfigureHiddenWindow is a no-op on non-Windows platforms: launching sh -c
+// from a desktop process does not create a new console window in the same way
+// Windows does.
+func ConfigureHiddenWindow(command *exec.Cmd) {}

src/platform/winproc/winproc_windows.go

@@ -0,0 +1,18 @@
+package winproc
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+// ConfigureHiddenWindow suppresses the console window that Windows would
+// otherwise flash when running a child process from a GUI application.
+// CREATE_NO_WINDOW keeps cmd.exe and simple console tools quiet while
+// stdout/stderr are still captured through pipes.
+func ConfigureHiddenWindow(command *exec.Cmd) {
+	if command.SysProcAttr == nil {
+		command.SysProcAttr = &syscall.SysProcAttr{}
+	}
+	command.SysProcAttr.CreationFlags |= 0x08000000
+	command.SysProcAttr.HideWindow = true
+}