feat: refactor plugin internals and stabilize isolation:none

[Lojhan]

Summary

• fix isolation none execution path by setting up runtime environment during plugin setup
• prevent duplicated metrics process listeners in shared-process runs
• refactor plugin internals into focused modules (plugin-command, plugin-metrics, plugin-setup, plugin-types)
• keep plugin.ts as orchestration layer and preserve public/internal behavior
• add focused tests for command and metrics modules
• update existing tests to async style consistency and include related benchmark/config updates from the current change set

Validation

• npm run test
• benchmark run command completed successfully from benchmark workspace (node ./run.mjs)