Add umbrella Gecode skill

[zayenz]

Summary

• add the merged umbrella gecode skill on top of the publishing infrastructure
• include the promoted always-on runtime model plus the debugging, modeling cookbook, set/float, and scheduling references
• intentionally exclude CMake guidance from this PR

Validation

• python3 scripts/validate_skills.py
• npx skills add . --list

Follow-up

• stacked PR Add CMake guidance to the Gecode skill #4 adds the CMake guidance layer only

[greptile-apps]

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