Add option to exclude specific dirs

[mhadidg]

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/**"