ci: package gateway stack platform releases

[captjt]

Summary

• teaches Helm CI to validate the new gateway-stack chart when synced from infrastructure
• validates the three Gateway image entries in consolidated release manifests
• exercises customer registry override rendering for Gateway images
• packages and publishes gateway-stack with the platform release

Compatibility

The manifest checks are conditional on charts.gatewayStack, so previously prepared release manifests remain supported.

Validation

• actionlint and YAML parsing for both changed workflows
• release-manifest branching reviewed for legacy and consolidated manifests
• git diff --check

Companion work

• https://github.com/eqtylab/governance-studio-infrastructure/pull/102
• https://github.com/eqtylab/guardian/pull/7