Tighten CI path triggers

[gabrimatic]

Summary

• switch product CI from broad ignores to explicit code-path triggers
• keep docs-only inputs on the Docs Pages workflow
• update the docs guard test for the stricter trigger contract

Verification

• flutter test test/docs_site_test.dart
• ruby -e 'require "yaml"; ARGV.each { |path| YAML.load_file(path); puts "#{path}: ok" }' .github/workflows/ci.yml .github/workflows/docs-pages.yml