ci: make OS real boot triggers future-proof

[fwerkor]

Summary

• replace the fragile build-path allowlist with a narrow non-build ignore list
• ensure vendored feeds, all packages, build scripts, tools, rootfs overlays, and future build inputs trigger real image builds
• keep documentation and repository-management-only changes from launching the six-hour build matrix

Validation

• actionlint .github/workflows/os-real-boot.yml
• YAML syntax parse
• the workflow file itself remains triggerable on both pull requests and pushes

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