Replace black and isort with ruff

[sandhose]

Summary

• Replace black (formatter) and isort (import sorter) with ruff format and ruff check --select I
• Update scripts-dev/lint.sh for ruff
• Update CI to use ruff check and ruff format --check (covering sydent/, tests/, stubs/, integration-tests/)
• Run ruff formatting and import sorting across the codebase

Test plan

• uv run ruff check sydent/ tests/ stubs/ integration-tests/ passes
• uv run ruff format --check sydent/ tests/ stubs/ integration-tests/ passes
• CI green

Part 3 of 9 in the modernisation series. Builds on #622.