docs: Add values.schema.json and NOTES.txt

[lfrancke]

Ports what hive-operator got in #743 and #753, so this is the second of sixteen. It adds three things, all documentation or validation — no behavioural change.

values.schema.json — Helm validates values against it on install, upgrade, lint and template, and Artifact Hub renders it as a values reference. additionalProperties is false, with the surface taken from what the templates actually read rather than from values.yaml.

NOTES.txt — no SDP chart has one, so helm install currently prints nothing. It also explains why no CRDs appear (they moved out of the chart, so kubectl get crd right after installing looks like a failed install), and warns when maintenance.customResourceDefinitions.maintain is false, where the chart ships no CRDs and the operator will not create them — currently a silent trap.

Four values documented in values.yaml — image.repository, image.productRepository, image.tag and priorityClassName are read by the templates but were absent from values.yaml. Since helm show values does not read the schema, they were invisible from the CLI, so someone pulling from a mirror had no way to discover image.repository.

Why the schema can be hive's with names substituted

The chart reads exactly the same values as hive's. Verified by extracting every .Values.* reference from every template in both charts and diffing: identical. values.yaml differs from hive's by one line, the header comment.

Verification

• default render byte-identical to main, so nothing changes for an existing install
• helm lint clean against both registry overlays, so the schema accepts the shipped values.yaml
• the schema really enforces: --set priorityClassNam=oops now fails with additional properties 'priorityClassNam' not allowed
• the four newly documented values take effect — image.tag reaches the image reference, productRepository reaches the IMAGE_REPOSITORY env var, priorityClassName reaches the pod spec
• NOTES.txt rendered with maintain both true and false
• both documentation links return 200