test(agent): isolate plugin agent regression

[kitlangton]

Summary

• Wire the plugin-agent regression test with explicit Effect layers instead of broad default layers
• Replace the real npm dependency-prep path with a fake Npm service so the test stays deterministic
• Keep the plugin config hook to Agent.list regression path covered

Test Plan

• bun run test test/agent/plugin-agent-regression.test.ts
• bun typecheck