Standardise installation instructions

[tom-leverstone]

Summary

Hi team! Nice work. I was following the installation instructions requiring git and cloning felt really odd. It's not a common practice in python (as far as I can tell, and not sure if it's common in other communities). Ideally, it would be nice to have this on PyPI. But with uv we can get quite far without as it can install and run directly from GitHub.

This PR does just that: adding an entry script and updating the README with the new running instruction.

AI usage

Claude Code wrote the change w/ Sonnet 4.6 to the prompt

The installation instructions on this repo are very much... non-pythonic. We don't need pypi if we don't want to, but we should be able to run the project without cloning with something like uvx --from git+https://github.com/OpenVanta/GrantGuard grantguard.

For that to work we need to change pyproject.toml slightly so grantguard is a command. Then we can update the README. Makes sense?

Testing

Verified locally by running verified by running uvx --from /Users/tomleverstone/code/GrantGuard grantguard`.

Related issue

None

Type of change

• Bug fix
• New risk detector / detection improvement
• New feature
• Docs / chore

Checklist

• uv run python -m compileall -q grantguard grantguard.py passes
• No new runtime dependencies (stdlib only)
• New secret-matching patterns are also covered by redact()
• Docs updated if behavior changed
• I verified the change manually (CLI and/or UI)