fix(plugins): respect PILOT_HOME paths

[MicroMilo]

Summary

• derive plugin directory and plugins.json from resolvePilotHome()
• make plugin storage honor PILOT_HOME like the rest of the UI server path helpers
• add regression coverage for isolated PILOT_HOME plugin paths

Verification

• npm --workspace ui exec vitest run server/utils/plugin-loader.pilot-home.test.js
• npm exec -- tsc --noEmit -p tsconfig.json

Notes

This is a real path consistency bug: plugin-loader hardcoded ~/.pilotdeck while ui/server/utils/pilotPaths.js already defines PILOT_HOME-aware home resolution.