feat: upgrade prometheus stack to v0.2.0 and parameterize installer

[yuanyp8]

Summary

• upgrade kube-prometheus-stack from 81.3.0 to 90.2.0
• add explicit component BOM in versions.env and docs/VERSION-BOM.md
• refresh offline image set for amd64/arm64
• standardize installer -h/--help and expose platform defaults as CLI parameters
• add repeatable --values-file for site-specific Helm overrides
• parameterize storage access modes, replicas, component toggles, discovery/dashboard labels, Grafana admin user, registry pull secret and related settings
• fix upgrade behavior: existing releases no longer run the destructive phase-1 disable/reenable sequence
• apply bundled CRDs before single-stage upgrades, with --skip-crd-upgrade escape hatch
• build the exact pinned upstream chart from OCI instead of manually maintaining the vendored chart copy
• make checksum files compatible with sha256sum -c
• strengthen CI with shell/help/image metadata/checksum validation

Target BOM

• kube-prometheus-stack 90.2.0
• Prometheus Operator 0.93.1
• Prometheus 3.14.0-distroless
• Alertmanager 0.34.0
• Grafana 13.2.1-distroless / chart 13.2.4
• kube-state-metrics 2.20.0 / chart 8.4.2
• node-exporter 1.12.1 / chart 4.57.0

Upgrade behavior

• first install: CRD/operator phase, then full stack
• existing release: CRD server-side apply, then one full Helm upgrade

Validation

CI builds both amd64 and arm64 offline installers and verifies generated checksums.