Refresh lint/format tooling

[snopoke]

Summary

• Bump ruff-pre-commit v0.2.1 → v0.15.12 and pre-commit-hooks v4.4.0 → v6.0.0 (both 2+ years stale)
• Update ruff hook id from ruff (legacy alias) to ruff-check per the newer ruff-pre-commit naming
• Apply lint findings the bumped ruff surfaces:
  • PT001: drop () from @pytest.fixture decorators (8 occurrences across tests/)
  • UP031: percent-format → f-string in taskbadger/utils.py:8
  • PT030: add match=... to a pytest.warns(DeprecationWarning) in test_project_key.py
  • UP007: Optional[X] → X | None in hand-written code (cli_main.py, cli/utils.py, mug.py)
  • UP038: isinstance(x, (A, B)) → isinstance(x, A | B) in celery.py
  • UP035: drop now-unused typing.Union import in mug.py
• Ignore UP* and F* rules in taskbadger/internal/ (generated by openapi-python-client; ~200 findings there are out of our hands)
• Drop black and isort from the dev dependency group — ruff and ruff-format (already in pre-commit) cover both

The Optional[X] and isinstance findings only appeared after #44 merged (which bumped ruff target-version to py310); they're rolled into this PR since the rebase needed them anyway.

Test plan

• `uv sync` resolves cleanly
• `uv run pytest -v` — 104 pass
• `uv run ruff check .` — all checks pass
• `pre-commit run --all-files` — all hooks pass

🤖 Generated with Claude Code