fix(daemon): cross-platform build for Dolt server management

Extract platform-specific syscall code into proc_unix.go and proc_windows.go
with appropriate build tags. This fixes TestCrossPlatformBuild which failed
because syscall.SysProcAttr.Setpgid is Unix-only.

Changes:
- setSysProcAttr(): Sets process group on Unix, no-op on Windows
- isProcessAlive(): Uses Signal(0) on Unix, nil signal on Windows
- sendTermSignal(): SIGTERM on Unix, Kill() on Windows
- sendKillSignal(): SIGKILL on Unix, Kill() on Windows

Fixes gt-3078ed

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/daemon/proc_unix.go

@@ -0,0 +1,32 @@
+//go:build unix
+
+package daemon
+
+import (
+	"os"
+	"os/exec"
+	"syscall"
+)
+
+// setSysProcAttr sets platform-specific process attributes.
+// On Unix, we detach from the process group so the server survives daemon restart.
+func setSysProcAttr(cmd *exec.Cmd) {
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Setpgid: true,
+	}
+}
+
+// isProcessAlive checks if a process is still running.
+func isProcessAlive(p *os.Process) bool {
+	return p.Signal(syscall.Signal(0)) == nil
+}
+
+// sendTermSignal sends SIGTERM for graceful shutdown.
+func sendTermSignal(p *os.Process) error {
+	return p.Signal(syscall.SIGTERM)
+}
+
+// sendKillSignal sends SIGKILL for forced termination.
+func sendKillSignal(p *os.Process) error {
+	return p.Signal(syscall.SIGKILL)
+}