fix(linters): forbid inert counts in v8 coverage-ignore comments

[secustor]

Changes

Adds a new custom oxlint rule renovate/v8-ignore-no-count that forbids the count form of coverage-ignore comments, e.g. /* v8 ignore next 4 -- foo */.

The count is not honored by the coverage provider: vitest's ast-v8-to-istanbul hint parser captures only the keyword (if/else/next/file), so next always exempts exactly the next AST node, however many lines it spans. A trailing count therefore suggests a line range that is never applied and misleads readers about what is actually exempted.

Details:

• New rule tools/lint/rules/v8-ignore-no-count.js, enabled as error in .oxlintrc.json. It is autofixable: the fixer drops the count (e.g. next 4 → next), which preserves the actual keyword-only behavior, so coverage results are unchanged.
• The autofix was applied to all 78 existing occurrences across lib/ and tools/.
• Updated the note in v8-ignore-reason.js that previously documented counts as "allowed inert noise" to point to the new companion rule.
• Added a bullet to docs/development/best-practices.md advising against counts.

Context

Please select one of the following:

• This closes an existing Issue, Closes: Add linting support which prevents next N in v8 code coverage comments #44728
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

The rule implementation, config wiring, and docs update were written by Claude Code (model: Claude Fable 5), which also ran the autofix over the existing occurrences. Verified via pnpm oxlint, pnpm type-check, pnpm biome, pnpm prettier, and targeted vitest runs.

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Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

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[jamietanna]

Conflicted otherwise LGTM