fix: update gitignore and add main entry point

- Fix gitignore to only ignore root-level gitauto binary
- Add cmd/gitauto/main.go entry point

cmd/gitauto/main.go

@@ -0,0 +1,7 @@
+package main
+
+import "github.com/iwasforcedtobehere/git-automation-cli/internal/cmd"
+
+func main() {
+	cmd.Execute()
+}