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.

[MicroMilo]

Validation note:

This PR fixes a real path consistency bug.

The UI server already has a shared resolvePilotHome() helper for resolving PilotDeck home state. Before this change, plugin-loader bypassed that helper and hardcoded plugin state under ~/.pilotdeck, including both the plugins directory and plugins.json.

That broke PILOT_HOME isolation for plugin state.

This PR changes plugin-loader to derive both paths from resolvePilotHome():

• plugin directory: <PILOT_HOME>/plugins;
• plugin config file: <PILOT_HOME>/plugins.json.

The regression test verifies this directly by setting PILOT_HOME to a temporary directory, writing plugin config, and reading it back from the same isolated home.

Validation commands used by this PR:

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

The scope is also clear: this PR fixes plugin-loader path resolution specifically. It does not claim that every UI/server path is now fully PILOT_HOME-clean.