Use explicit values for Alloy to Cortex Tenant CI

General MR

Summary

• Replace the removed umbrella alloy.cortexTenant key in package CI with explicit Alloy destination, tenant label, network-policy, and HelmRelease dependency values.
• Keep the Prometheus-disabled exporter scrape path covered by the package integration test.
• Correct package guidance about the Mimir ingress rule, which requires explicit opt-in.

Relevant logs/screenshots

• helm lint ./chart and standalone Helm render passed.
• The package CI overlay rendered against umbrella branch alloy-cortex-tenant-prewiring; Alloy selected only Cortex Tenant and Mimir distributor ingress resolved to true.
• Clean-install integration CI remains the cross-package validation.

Linked Issue

big-bang/bigbang#3464 (closed)

Upgrade Notices

N/A; this changes package CI values and documentation only.