feat: port chart-releaser release pipeline from helm-template

[fty4]

Summary

• Ports helm-template's release.yaml chart-releaser pipeline into taskmedia/helm, adapted for the multi-chart charts/ directory.
• Incorporates the findings from Research: pre-flight facts for adopting chart-releaser-action #2's pre-flight research (docs/research/chart-releaser-cutover.md on research/chart-releaser-cutover):
  • Keeps the fty4/helm_chart-releaser-action@v1.8.1-rc1 fork — upstream helm/chart-releaser#587 is still unmerged.
  • Relies on the existing job-level permissions: contents: write — repo's default_workflow_permissions is already write, no Settings change needed.
  • Leaves the "bump every chart's version before the first run" cutover step as an operational action (tracked for Verify the new release pipeline end-to-end #4), not workflow logic.
• Key adaptation vs. the source: taskmedia/helm has no README.md on main to mirror into gh-pages (unlike helm-template), so the README chart-table step edits gh-pages' own README.md in place and self-bootstraps the <!-- start charts -->/<!-- end charts --> markers on first run instead of mirroring/skipping.
• .github/workflows/test.yaml is left untouched — it already carries every relevant improvement from helm-template's lint-test.yaml (newer action pins, --target-branch, --check-version-increment=false, --chart-dirs charts).

Test plan

• actionlint .github/workflows/release.yaml passes clean.
• Merge, then verify end-to-end per Verify the new release pipeline end-to-end #4: bump a chart version, confirm a GitHub Release + index.yaml entry is created, the gh-pages README chart table updates, and CNAME/artifacthub-repo.yml on gh-pages are untouched.

Closes #3