ci: add platform-owned reusable workflows

[NWarila]

Summary

• Add platform-owned reusable workflows for repo hygiene, CodeQL, IaC security, Scorecard, auto-merge, and release-please.
• Add repo-hygiene OPA/tooling baseline and self-CI smoke coverage.
• Align reusable references with the nwarila-platform/.github control plane.

Validation

• python tools\build_opa_input.py
• Workflow YAML parsed with PyYAML
• rg advisory/continue-on-error returned no matches in reusable-repo-hygiene.yaml

Rollout

Platform consumers pin commit 26473a3.

[github-advanced-security]

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