fix(skills): normalize command resource paths

[cbusillo]

Summary

• normalize skill command resource paths before matching declared resources
• keep missing normalized helper paths on the file-existence error path instead of undeclared-resource errors
• fetch upstream cursor refs with --no-tags to avoid upstream/product tag collisions

Validation

• cd code-rs && cargo test -p code-core skills::loader::tests
• scripts/local/upstream-cursors.sh --no-fetch
• rg "git fetch --no-tags" scripts/local/upstream-cursors.sh
• ./build-fast.sh