Migrate dev tooling from pip-tools/tox to uv

[hf-krechan]

Migrate this repo's dev tooling from pip-tools/tox to uv, using the
migrating-pip-tools-tox-to-uv skill.

• pyproject.toml: dev-tool extras moved to [dependency-groups]
• uv.lock generated, tox.ini/requirements.txt removed
• CI workflows rewritten to use astral-sh/setup-uv + uv run/uv sync
• .github/dependabot.yml ecosystem updated to "uv"
• README / .pre-commit-config.yaml updated
• PR opened and CI green

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[hf-krechan]

Migration complete: #257