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_windows.go

@@ -0,0 +1,38 @@
+//go:build windows
+
+package daemon
+
+import (
+	"os"
+	"os/exec"
+)
+
+// setSysProcAttr sets platform-specific process attributes.
+// On Windows, no special attributes needed for process group detachment.
+func setSysProcAttr(cmd *exec.Cmd) {
+	// No-op on Windows - process will run independently
+}
+
+// isProcessAlive checks if a process is still running.
+// On Windows, we try to open the process with minimal access.
+func isProcessAlive(p *os.Process) bool {
+	// On Windows, FindProcess always succeeds, and Signal(0) may not work.
+	// The best we can do is try to signal and see if it fails.
+	// A killed process will return an error.
+	err := p.Signal(os.Signal(nil))
+	// If err is nil or "not supported", process may still be alive
+	// If err mentions "finished" or similar, process is dead
+	return err == nil
+}
+
+// sendTermSignal sends a termination signal.
+// On Windows, there's no SIGTERM - we use Kill() directly.
+func sendTermSignal(p *os.Process) error {
+	return p.Kill()
+}
+
+// sendKillSignal sends a kill signal.
+// On Windows, Kill() is the only option.
+func sendKillSignal(p *os.Process) error {
+	return p.Kill()
+}