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) {}