feat: add offline repository scanning

[techgaun]

Summary

• add --local PATH for scanning cloned repositories, source trees, or individual files without GitHub API access
• use Git-tracked and non-ignored files when possible, with a dependency-free filesystem fallback
• translate the project dictionaries into local predicates supporting quoted text, OR, NOT, filename:, path:, extension:, and common language: qualifiers
• reuse text, CSV, JSON, and JSONL result writers for local findings
• include matching line snippets, relative paths, file URLs, and local file counts
• skip binary files, symlinks, oversized files, and the scanner output file
• add --max-file-size with a 1 MB default
• document offline usage and current working-tree scope

Closes #22. Git history scanning remains a possible follow-up; the original issue described it as optional.

Verification

• python3 -m unittest discover -s tests -v (35 tests)
• parsed all 186 bundled dorks successfully
• python3 -m py_compile github_dorks/local.py github_dorks/cli.py
• git diff --check
• built the Python 3.12 Docker image
• ran github-dorks --local /src --category ai --format jsonl --quiet in the built image