adopt ruff: pinned formatter + import sorting, opt-in pre-commit hook

PR #1 arrived black-formatted, which forced the question. ruff's formatter
is black's output, it is one static binary rather than a dependency chain,
and it is fast enough that a hook is free. ruff.toml pins the version
(a formatter whose output moves between releases turns 'formatted' into
'formatted by whoever committed last') and selects only import sorting on
the lint side -- a hook that refused a commit over an unused variable would
be a different feature.

.githooks/pre-commit (install: git config core.hooksPath .githooks) formats
exactly what is staged, refuses partly-staged files rather than quietly
widening a commit built with git add -p, and prefers the pinned .venv ruff.

Also drops the blanket '# ruff: noqa' PR #1 left in remote_ops.py -- it
would have exempted that file from the only lint rule we enforce.

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