feat: add enable_helm input

[hisco]

Summary

• Adds enable_helm input (default true) that passes --enable-helm to kustomize build.
• When true, fails fast if the helm binary is not on the runner with a clear actionable error.
• Lets overlays using helmCharts: work out of the box; can be set to false for runners without helm.

Test plan

• Run on overlay using helmCharts: with helm installed - inspect succeeds.
• Run with enable_helm: false on a non-helm overlay - behaves as before.
• Run with enable_helm: true on runner without helm - fails fast with clear error.

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Note

Low Risk
Low risk: small, opt-in build flag change plus early tool validation; main impact is CI behavior now requiring helm when enable_helm=true (default).

Overview
Adds a new enable_helm input (default true) that conditionally passes --enable-helm to kustomize build, enabling overlays that use helmCharts:.

When enabled, the action now fails fast during tool checks if the helm binary is not available, and the build step was updated to safely construct/expand the optional flag array. Documentation was updated to describe the new input and its behavior.

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