ci: single-source the Python version via pyproject.toml + add uv-lock pre-commit hook

[priya-sundaram-dev]

As discussed in #15081 and #15104 (comment), this does the two follow-ups:

1. Add astral-sh/uv-pre-commit (uv-lock hook)

Keeps uv.lock automatically in sync with pyproject.toml. Right now the lockfile only gets refreshed by hand, so it can silently drift (e.g. after a dependency bump). With this hook, pre-commit.ci relocks and auto-commits whenever pyproject.toml changes, so uv sync in CI always sees a consistent lock.

- repo: https://github.com/astral-sh/uv-pre-commit
  rev: 0.12.7
  hooks:
    - id: uv-lock

2. python-version-file: pyproject.toml on every actions/setup-python

Replaces the hard-coded python-version: 3.14 in build.yml, project_euler.yml (both jobs), sphinx.yml, and directory_writer.yml with python-version-file: pyproject.toml. actions/setup-python@v7 reads project.requires-python (>=3.14), so the interpreter version now has a single source of truth — bumping requires-python moves all of CI at once and there's no chance of a workflow being left behind.

No behavioural change today (still resolves to 3.14); this is a maintainability cleanup. Happy to adjust the pin style if you'd prefer a .python-version file instead.

[algorithms-keeper]

Closing this pull request as invalid

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[cclauss]

Perfect!