chore: harden GitHub Actions

[rsclarke]

Hardens the repository's GitHub Actions surface with least-privilege permissions, safer checkout behavior, Dependabot cooldowns, and ongoing zizmor auditing. This also adds Taskfile shortcuts so local zizmor audits and safe fixes can be run consistently.

• Reduce CI token exposure by setting workflow-level empty permissions, granting only read access for the test job, and disabling checkout credential persistence.
• Add Dependabot cooldowns for Go module and GitHub Actions updates.
• Address zizmor auditor findings by adding CI concurrency limits and an explicit test job name.
• Add a pinned zizmor audit workflow that uploads SARIF to GitHub code scanning.
• Add task zizmor and task zizmor:fix shortcuts.

[github-advanced-security]

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