Summary

Updates the Iceberg experimental skill to use UniForm Iceberg Compat V3 as the positive path and DBR 18.0+ as the runtime guidance.

Changes

• Replace V2-positive examples with delta.enableIcebergCompatV3
• Add V2-to-V3 upgrade guidance that disables V2 explicitly
• Update managed Iceberg runtime text to DBR 18.0+
• Add content guards to prevent V2/DBR 17.3 regressions

Test Plan

• python3 scripts/skills.py validate
• python3 -m unittest discover -s tests -p '*_test.py'
• LSP diagnostics clean on the new content test

Fixes #159