Align Helm chart release automation

[michaelw]

Summary

• align CI and Release Please workflows with ext-authz-token-exchange conventions for a Helm library chart
• publish common.itsumi under ghcr.io/michaelw/charts and preserve component-prefixed release tags
• add fixture and Copier starter-template validation plus Renovate config and docs

Validation

• pre-commit run --all-files
• helm dependency build .
• helm lint .
• helm dependency build test/fixtures/common-itsumi-fixture
• helm template common-itsumi-fixture test/fixtures/common-itsumi-fixture --namespace common-itsumi
• Copier starter generation, dependency build, lint, and template render
• workflow YAML parse check