Add interrogate docstring coverage checks

Add interrogate to this repository and integrate it with existing tox, pre-commit, and CI style checks. Scope: configure interrogate, wire it into quality tooling, document local usage, and avoid src changes.

Start by locating the repository's tox, pre-commit, CI, and documentation configuration, then review how existing style checks are wired together. Configure interrogate consistently across those quality checks and document local usage; done means the checks run successfully without changes under src.

Written by the indexing model from the issue text.