[chore] Move ruff select/ignore under [tool.ruff.lint]

[SriRangaTarun]

Summary

Ruff deprecated top-level select/ignore in [tool.ruff] in favor of the
[tool.ruff.lint] section. With the currently pinned toolchain every ruff
invocation — local, pre-commit, and CI — emits:

warning: The top-level linter settings are deprecated in favour of their
counterparts in the `lint` section.

Since .pre-commit-config.yaml pins ruff-pre-commit and pre-commit.ci
autoupdates quarterly, this will keep resurfacing until the legacy path is
removed upstream, at which point the lint configuration would silently stop
applying.

Change

Moves select and ignore into a new [tool.ruff.lint] section. line-length
stays at the top level, where it still belongs.

Verification

The enabled rule set is unchanged — ruff check --show-settings produces an
identical linter.rules.enabled block before and after. ruff check . passes
with no findings and no deprecation warning.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.