chore: fix npm caching in linter workflow

[Copilot]

npm ci deletes node_modules before installing, making the previous actions/cache step that cached node_modules a no-op — restoring it provided zero speedup.

Changes

• Remove the standalone actions/cache@v5 step caching node_modules
• Add cache: 'npm' to actions/setup-node@v6, which caches ~/.npm keyed by package-lock.json — the correct layer to cache for npm ci workflows

- name: "Setup Node.js"
  uses: actions/setup-node@v6
  with:
    node-version: '22.x'
    cache: 'npm'

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[mobilemind]

Looks good to me.

[Copilot]

Pull request overview

Updates the linter GitHub Actions workflow to use the built-in npm cache support in actions/setup-node instead of caching node_modules, aligning caching behavior with npm ci semantics.

Changes:

• Removed the standalone actions/cache step that cached node_modules.
• Enabled actions/setup-node@v6 npm caching (cache: 'npm') keyed off package-lock.json.

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