fix: use active node npm for wrapper updates

[cafitac]

Summary

• make agent-learner update prefer the npm executable next to the active node
• fall back to plain npm only when no sibling executable exists
• add a regression test covering the active-node npm selection

Problem

When the wrapper shelles out to bare npm, non-interactive/login shells can resolve a different Node/npm tree than the active wrapper binary. In this environment that caused agent-learner to live under ~/.nvm/... while npm install -g targeted /usr/local, leading to permission errors and inconsistent updates.

Test Plan

• node --test test/wrapper.test.cjs
• npm test