proposal: add `prek lint`

Add a lightweight prek lint command for checking config/manifest issues and hosting future lint rules.

prek lint is intended to replace prek validate-config and prek validate-manifest as the primary validation entry point. The existing commands can remain as compatibility aliases/wrappers for the corresponding lint checks.

Expected rules

Rule	Mode	Default	Fixable	Purpose	Issue
unknown-keys	offline	yes	no	Warn about ignored config keys	n/a
mutable-rev	offline	yes	no	Warn about branch/moving revs	n/a
ineffective-regex	offline	yes	no	Warn about regexes that likely do not match as intended	#433
legacy-stages	offline	yes	yes	Warn about deprecated stage names (commit, merge-commit, push) and migrate them to pre-commit, pre-merge-commit, pre-push	n/a
ref-version-mismatch	online	yes	yes	Check SHA rev matches the # frozen: version comment	#1864
unpinned-rev	offline	opt-in	no	Require remote repo rev to be hash-pinned	#2146
unsupported-hook-options	offline	yes	no	Statically report hook options that the selected language/repo kind does not support. e.g language: system with additional_dependencies.	n/a

Future rule ideas

These are possible future lint rules, not necessarily part of the initial implementation.

Rule	Mode	Default	Fixable	Purpose	Issue
stale-repo-url	online	maybe	maybe	Detect GitHub repo URL redirects / canonical URL mismatch	#1891
ambiguous-rev	offline	maybe	yes	Warn about YAML rev values that look numeric/bool-like and should be quoted	#866, #1935, #1343
useless-excludes	offline	maybe	no	Check exclude patterns that do not match any files	pre-commit meta hook
unmatched-hooks	online	maybe	no	Check hooks that do not apply to any file in the repo	pre-commit meta hook
unknown-hook-ids	online	maybe	no	Check configured hook ids exist in the remote manifest at the pinned rev	n/a
version-drift	offline/online	maybe	no	Detect hook versions drifting from project lockfile/tool versions	#1054
complex-regex	offline	maybe	maybe	Suggest regex-list style once regex lists exist	#2250