ci: add helm lint workflow triggered on helm chart changes

[TaylorMutch]

Summary

• Adds a dedicated helm-lint.yml workflow that runs mise run helm:lint on PRs that touch deploy/helm/**
• Path filter on the trigger means the workflow is skipped entirely for unrelated PRs
• Uses the same pr_metadata gate and CI image as the other branch-checks workflows

Related Issue

Stacked on #1159 (which introduces the helm:lint matrix task and the ci/ values directory).

Changes

• .github/workflows/helm-lint.yml — new workflow: triggers on deploy/helm/** path changes, runs mise run helm:lint in the CI container

Testing

• Verify workflow triggers on a PR with helm chart changes
• Verify workflow is skipped on PRs with no helm chart changes

Checklist

• Follows conventional commits format
• No secrets or credentials committed

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[TaylorMutch]

/ok to test d605eab

[TaylorMutch]

/ok to test 6b6b0a5