fix(nix): add venv setup to dev install docs

[ruthwikdasyam]

Summary

• Added python3 -m venv .venv and source .venv/bin/activate to the "Developing on DimOS" section in the Nix install docs

Why

Without a venv, pip install in a Nix shell attempts to write to the read-only Nix store Python, causing failures. The "Using DimOS as a library" section already had this step, but it was missing from the dev setup.

[greptile-apps]

Greptile Summary

This PR adds the necessary virtual environment setup commands (python3 -m venv .venv and source .venv/bin/activate) to the "Developing on DimOS" section of the Nix installation documentation. This change ensures consistency with the "Using DimOS as a library" section, which already included these commands. The change is necessary because without a virtual environment, pip install attempts to write to the read-only Nix store Python directory, causing installation failures.

Confidence Score: 5/5

• This PR is safe to merge with no risk
• The changes are documentation-only, adding two essential commands that were already present in another section of the same file. The commands are correct for Nix environments where pip requires a venv to avoid write conflicts with the read-only Nix store. The .gitignore file confirms .venv* directories are already ignored, making this change fully compatible with the existing setup.
• No files require special attention

Important Files Changed

Filename	Overview
docs/installation/nix.md	Added venv setup commands to "Developing on DimOS" section for consistency with library usage section

_{Last reviewed commit: e45ee68}

[greptile-apps]

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