Add option to exclude specific dirs

Currently, the tool scans and renders all files in the repo (except `.git/`, binaries, large files, and symlinks). In practice, many repos contain datasets, generated data, tests, or dependency dirs that add noise but no useful context. This adds unnecessary tokens for metered queries (OpenAI, Anthropic, etc.), which are charged per input token.

### Proposed solution:

Introduce a new option that allows users to specify dirs (or filepath patterns) to skip.

```
# either explicit dirs
rendergit https://github.com/example/project --exclude-dirs tests

# or glob pattern, repeatable option
--exclude "tests/**"
```

Start at the rendergit CLI entry point and trace how repository paths are selected before rendering. Implement the proposed --exclude-dirs or repeatable --exclude glob option, then verify that matching directories and file paths do not appear in the rendered output.

Written by the indexing model from the issue text.