fix(cmd): extract orphan cleanup to platform-specific files (#835)

PR #759 introduced cleanupOrphanedClaude() using syscall.Kill directly,
which breaks Windows builds. This extracts the function to:
- start_orphan_unix.go: Full implementation with SIGTERM/SIGKILL
- start_orphan_windows.go: Stub (orphan signals not supported)

Follows existing pattern: process_unix.go / process_windows.go

internal/cmd/start_orphan_windows.go

@@ -0,0 +1,16 @@
+//go:build windows
+
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/steveyegge/gastown/internal/style"
+)
+
+// cleanupOrphanedClaude is a Windows stub.
+// Orphan cleanup requires Unix-specific signals (SIGTERM/SIGKILL).
+func cleanupOrphanedClaude(graceSecs int) {
+	fmt.Printf("  %s Orphan cleanup not supported on Windows\n",
+		style.Dim.Render("○"))
+}