Split lint into separate workflow

[adamtheturtle]

Split lint into a separate lint.yml workflow so that adding hook-stage to the matrix doesn't cause tests to run 3x redundantly.

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Note

Low Risk
Workflow-only refactor with no runtime code changes; main risk is misconfigured GitHub Actions triggers/matrix causing lint or status checks to run unexpectedly or not at all.

Overview
Splits GitHub Actions so ci.yml only runs the pytest matrix (Python 3.13/3.14 on Ubuntu/Windows), removing the hook-stage dimension and the inline Lint step to avoid redundant test runs.

Adds a new lint.yml workflow that runs prek across pre-commit/pre-push/manual hook stages (Python 3.13 on Ubuntu/Windows) and includes a completion job to fail the workflow if any matrix leg fails, mirroring the CI completion gating.

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