docs(skill-upper): add YAML field comments

[googs1025]

Summary

• add field-leading comments to skill-upper eval and case YAML templates
• clarify generated YAML comment language rules while preserving technical identifiers
• extend skill-upper scaffold eval checks for English-only output and key field comments

Verification

• ruby -e 'require "yaml"; YAML.load_file("skills/skill-upper/assets/eval.yaml.tmpl"); YAML.load_file("skills/skill-upper/assets/case.yaml.tmpl"); puts "PASS templates"'
• GOTOOLCHAIN=go1.26.4 make verify
• GOTOOLCHAIN=go1.26.4 GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=commit.gpgsign GIT_CONFIG_VALUE_0=false make test

Closes #174