ci: publish charts to a classic index-based Helm registry

The publish step no longer speaks OCI. "helm registry login", "helm push"
and "helm registry logout" are replaced by a single HTTP POST of the
packaged .tgz to <charts-url>/api/charts, with Basic auth per request.

Credentials and the target URL come from a new 1Password item
op://GitHub-Actions/jlab-registry-access (username, password, charts-url).
The old item is left untouched, so a revert of this commit restores the
previous pipeline without any further work. No registry address enters
the repository.

The three properties of the old step survive:

- values are read via steps.<id>.outputs.* into the step's own env block
  and used as quoted shell variables, each guarded with ":?"
- credentials stay out of argv: "curl --config -" instead of "--user"
- a failed upload fails the job: "--fail-with-body", because curl exits 0
  on HTTP 401/403/409 without it and the publish would go green with
  nothing uploaded

The "<chart>-*.tgz" glob is replaced by the file name built from the
chart name and the version already read from Chart.yaml; --upload-file
takes exactly one file. Two small additions fall out of measuring the
new step: the registry host is masked, because a curl connection error
prints only "<host>:<port>" and the mask on the full URL does not cover
that, and whitespace in the credentials is rejected up front, because an
unquoted curl config value ends at the first space.

No chart is touched and no chart version is bumped.

CLAUDE.md

@@ -26,7 +26,7 @@ Additional rules for all commits, PRs, issues, and comments:
 
 ## What this repo is
 
-A collection of self-made Helm charts, one directory per chart, published to a private OCI registry via GitHub Actions. Validation happens on three levels: locally with Helm, in the PR pipeline (`validate-charts.yml`: strict lint, package, version-bump check), and in the PR install-test stage (k3d, fixtures under `ci/` — see README):
+A collection of self-made Helm charts, one directory per chart, published to a private, classic (index-based) Helm registry via GitHub Actions — an HTTP `POST` of the packaged `.tgz`, not an OCI `helm push`; the target URL and its credentials live in 1Password and never in this repository. Validation happens on three levels: locally with Helm, in the PR pipeline (`validate-charts.yml`: strict lint, package, version-bump check), and in the PR install-test stage (k3d, fixtures under `ci/` — see README):
 
 ```bash
 helm lint --strict <chart-name>