chore: Add Dependabot version-update cooldown

[ld-repository-standards]

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• Ensure every entry under updates in .github/dependabot.yml declares a cooldown of at least 7 days (default-days).
• Add entries for detected package ecosystems that were not yet tracked by Dependabot.

Cooldown applies only to version updates; security updates bypass it, so critical CVE fixes are never delayed.

Ref: SEC-8058.

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Note

Low Risk
CI/dependency automation only; no application runtime or auth logic changes.

Overview
Introduces .github/dependabot.yml to enable weekly Dependabot updates for github-actions and npm, each with a cooldown.default-days: 7 so version bumps are not opened back-to-back.

Per the PR intent, this satisfies SEC-8058: ecosystems get explicit tracking and version-update spacing; security updates are unaffected by the cooldown.

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