Add a Helm value

The rules are in helm/AGENTS.md.

1. Write the unit test first under helm/dexaflow/tests/: what the template renders with the value set, and what it renders without it. config_bind_test.yaml is the pattern for a value that becomes a server environment variable. Run helm unittest helm/dexaflow and watch it fail.

2. Add the key to helm/dexaflow/values.yaml with a # -- doc comment above it. Its default keeps the rendered manifests of an existing install unchanged.

3. Render it in the template that uses it. A server setting reaches the control plane as an environment variable in templates/_controlplane-deployment.tpl; the server reads it under either prefix (see add a configuration setting). When a combination of values would make the server refuse to boot, fail the render with a message that says what to set, as templates/deployment.yaml does for its guarded combinations.

4. Regenerate the README from README.md.gotmpl, never by editing the table:

   helm-docs -c helm/dexaflow
   bash scripts/check-values-doc-comments.sh   # what helm-docs cannot see
   

5. Run the chart checks:

   helm lint helm/dexaflow
   helm unittest helm/dexaflow
   bash scripts/helm-template-checks.sh
   bash scripts/rbac-covers-executor.sh        # when the Role or the executor changed
   

   Helm CI also installs the previous released chart and upgrades it to yours; a renamed value keeps the old key working.

6. Leave Chart.yaml alone. The release cut bumps version and appVersion together with the tag (scripts/check-chart-version-matches-tag.sh).

7. Document it. values.yaml is user-facing surface for the docs guard: update the Helm chart page or the page of the feature, and record a changelog fragment (record a change).