Enforce shebang format on command files

[codebymikey]

The Issue

• Fixes Checker should detect if the command file does not start with a shebang #106

Shebangs aren't properly enforced for command files.

How This PR Solves The Issue

Checks if the first line of command is a shebang.

Manual Testing Instructions

Inside a DDEV addon like https://github.com/FreelyGive/ddev-claude-code or equivalent:

printf '#ddev-generated\n#!/bin/bash' > commands/web/test
curl -fsSL https://raw.githubusercontent.com/codebymikey/ddev-addon-template/refs/heads/patch-1/.github/scripts/update-checker.sh | bash

Automated Testing Overview

N/A

Release/Deployment Notes

N/A

[stasadev]

Thank you!