feat: backstage-override package for seamless local stack setup

[punkwalker]

Summary

Adds a backstage-override idpbuilder package that replaces the ref-implementation's default Backstage deployment, eliminating the manual image swap and ArgoCD revert loop.

What changed

• examples/local-stack/backstage-override/ — Full Backstage deployment manifest with CI-built image, guest auth, NODE_OPTIONS, homepage quick links, and k8s config baked in
• examples/local-stack/backstage-override.yaml — ArgoCD Application named backstage that overrides the ref-implementation's default
• Taskfile.yml — task stack:up is now a single command: idpbuilder + keycloak fix + kro + patches
• examples/local-stack/README.md — Updated with quick start and troubleshooting

Problems solved

1. ArgoCD image revert loop — Override package owns the backstage ArgoCD app, so no more sync conflicts
2. Keycloak secret bug — stack:keycloak-fix task automatically extracts secrets from job logs
3. Manual image swap — CI image is baked into the manifest, no kubectl set image needed

Usage

task stack:up      # Full stack from scratch
task stack:iterate # Fast rebuild after code changes
task stack:down    # Tear down

Test Plan

• task --list shows all stack commands
• Verified backstage-override manifest matches ref-implementation structure
• CI image tag matches current main HEAD