ci(publish): compose the registry URL from a base and the repository owner

charts-url in op://GitHub-Actions/jlab-registry-access held the full
registry URL and publish-helm-chart.yml appended /api/charts to it. It
now holds only the base up to /api/packages, and the owner segment comes
from github.repository_owner, so the target is composed as
<base>/<owner>/helm/api/charts.

Nothing in this repository points at the registry any more, not even a
path segment, and the second owner on the same instance needs no second
setting. The 26 thin publish-<chart>.yml callers are unchanged.

Two guards keep the switch from the old shape loud rather than a 404
against a masked URL: charts-url is rejected unless it ends in
/api/packages, naming /helm, /api/charts or an included owner
explicitly; and the derived owner is rejected when empty or when it is
not a usable URL path segment. Everything the upload step already had
is untouched - the per-field :? guards, the whitespace check on the
credentials, the host masking, the built file name, --fail-with-body.

The probe step in validate-charts.yml is temporary and measures whether
this forge populates github.repository_owner at all; it is removed again
once the value has been read off the run.

.github/workflows/validate-charts.yml

@@ -14,6 +14,16 @@ jobs:
       install-charts: ${{ steps.detect.outputs.install-charts }}
       any-install: ${{ steps.detect.outputs.any-install }}
     steps:
+      # TEMPORARY (#63) - remove before the PR is merged. Measures whether this
+      # forge populates github.repository_owner at all; the publish URL is about
+      # to be composed from it.
+      - name: "TEMPORARY (#63): probe github.repository_owner"
+        env:
+          PROBE_OWNER: ${{ github.repository_owner }}
+        run: |
+          echo "github.repository_owner = [${PROBE_OWNER}]"
+          echo "github.repository       = [${{ github.repository }}]"
+          echo "length = ${#PROBE_OWNER}"
       - name: "Checkout Repository"
         uses: actions/checkout@v7
         with: