ci: reduce develop pull request checks

[kexi]

Summary

• keep develop pull requests focused on lightweight required checks
• move full multi-platform distribution validation to pushes and main-bound pull requests
• keep the aggregate CI check strict about unexpected skipped jobs

Tests

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml"); puts "ok"'
• pinact run --check .github/workflows/ci.yml
• pnpm run fmt:check
• pnpm run lint
• pnpm run check:all