Make base path portable for domain migration

[braboj]

As the site owner migrating to codewithbranko.com, I want the base path to be configurable without editing source.

What

astro.config.mjs line 9 hardcodes base: '/tutorial-git/'. Also public/robots.txt hardcodes the sitemap URL to braboj.me.

Why

Domain migration (#135) will require changing these values. An env variable makes it portable.

Acceptance criteria

• Base path reads from environment variable with fallback to /tutorial-git/
• robots.txt auto-generated from Astro config site value (not hardcoded)