Support GitHub URLs with branch or subdirectory context #164

The current generator assumes a simple repository root URL. Monorepos and branch-specific URLs would benefit from preserving the original context when a user shares a deeper GitHub path.

Proposed scope

• Accept URLs that include tree/<branch> or nested repository context.
• Extract repository metadata while keeping branch context available to generation logic.
• Avoid rejecting valid GitHub repository URLs that are not root-only.

Acceptance criteria

• Valid GitHub URLs with branch context are accepted.
• The generator still resolves the correct repository owner and name.
• Invalid non-repository GitHub paths continue to fail cleanly.