Add repository hygiene checks

[CaspianG]

What changed

• add Ruff to the development toolchain and GitHub Actions
• remove unused imports, variables, and duplicate public exports
• preserve intentional late imports in standalone benchmark and example scripts
• add httpx2 for the current Starlette TestClient while retaining direct httpx process tests

Why

A full repository audit found no secrets, broken local documentation links, stale benchmark artifacts, or failing tests, but static linting was not enforced and 18 actionable findings had accumulated. This change makes repository hygiene reproducible in CI.

Validation

• pytest -q: 743 passed, 5 skipped before cleanup
• targeted regression suite: 118 passed
• warning regression check: 1 passed with no warnings
• ruff check wavemind tests benchmarks examples: passed
• benchmark artifact audit: passed
• package build and twine check: passed
• pip check: passed