feat: status command + windows toast notifications

- status: read-only report of deployed/repository revision, pending changes,
  skipped items (with reasons), unrecognized local items, binary version;
  --json machine-readable; exit 1 when local modifications block repository
  updates (headless drift checks)
- sync/status share a read-only prepare() front half
- notify package: windows toast via PowerShell script (go-toast mechanism,
  zero deps, build-tag gated), noop on linux/macos; cross-compile verified
- toast fires exactly once per sync with warnings, never for routine syncs;
  notification failures are logged and never fail the sync (fail-open)

internal/notify/toast_windows.go

@@ -0,0 +1,49 @@
+//go:build windows
+
+package notify
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// PowerShellToast raises a Windows toast notification by invoking
+// PowerShell with an AppId-tagged script — the same mechanism go-toast
+// uses, without the dependency.
+type PowerShellToast struct {
+	AppID string
+}
+
+// Platform returns the PowerShell toast sender on Windows.
+func Platform() Sender { return PowerShellToast{AppID: "agent-lib"} }
+
+var toastScript = `$app = '%[1]s'
+[Windows.UI.Notifications.ToastNotificationManager, Windows.UI.Notifications, ContentType = WindowsRuntime] | Out-Null
+$tmpl = [Windows.UI.Notifications.ToastNotificationManager]::GetTemplateContent([Windows.UI.Notifications.ToastTemplateType]::ToastText02)
+$txt = $tmpl.GetElementsByTagName('text')
+$txt.Item(0).AppendChild($tmpl.CreateTextNode('%[2]s')) | Out-Null
+$txt.Item(1).AppendChild($tmpl.CreateTextNode('%[3]s')) | Out-Null
+$toast = [Windows.UI.Notifications.ToastNotification]::new($tmpl)
+[Windows.UI.Notifications.ToastNotificationManager]::CreateToastNotifier($app).Show($toast)`
+
+// Send fires one toast. It requires an interactive user session; callers
+// must treat errors as non-fatal.
+func (p PowerShellToast) Send(title, body string) error {
+	appID := p.AppID
+	if appID == "" {
+		appID = "agent-lib"
+	}
+	script := fmt.Sprintf(toastScript, escape(appID), escape(title), escape(body))
+	cmd := exec.Command("powershell", "-NoProfile", "-NonInteractive", "-Command", script)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("toast failed: %v: %s", err, strings.TrimSpace(string(out)))
+	}
+	return nil
+}
+
+// escape makes s safe inside a PowerShell single-quoted string.
+func escape(s string) string {
+	r := strings.NewReplacer("'", "''", "\n", " ", "\r", " ")
+	return r.Replace(s)
+}