Fix .gitignore ignoring cmd/syncwarden directory

The pattern 'syncwarden' matched cmd/syncwarden/ too.
Anchor binary patterns to repo root with leading slash.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/syncwarden/main.go

@@ -0,0 +1,9 @@
+package main
+
+import (
+	"git.davoryn.de/calic/syncwarden/internal/tray"
+)
+
+func main() {
+	tray.Run()
+}