fix: Add Windows build support for daemon command

- Split platform-specific daemon process configuration into separate files
- daemon_unix.go: Uses Setsid for Unix/Linux/macOS
- daemon_windows.go: Uses CREATE_NEW_PROCESS_GROUP for Windows
- Fixes compilation error: "unknown field Setsid in struct literal"

This allows bd.exe to build successfully on Windows while maintaining
proper daemon behavior on all platforms.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/daemon_windows.go

@@ -0,0 +1,16 @@
+//go:build windows
+
+package main
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+// configureDaemonProcess sets up platform-specific process attributes for daemon
+func configureDaemonProcess(cmd *exec.Cmd) {
+	// Windows doesn't support Setsid, use CREATE_NEW_PROCESS_GROUP instead
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		CreationFlags: syscall.CREATE_NEW_PROCESS_GROUP,
+	}
+}