fix: non-interactive npx for skills install (postinstall + learn)
Both postinstall.js (npm install hook) and cmd/learn.js (vit learn)
spawn `npx skills add` via spawnSync. The trailing -y goes to the
skills CLI, not to npx, so on a fresh machine without cached
skills@X.Y.Z, npx halts with an interactive "Ok to proceed?" —
blocking npm install and any coding-agent-driven skill learn.
Pass --yes to npx directly (suppresses the prompt) and set CI=true in
env as belt-and-suspenders against any other TTY-gated checks in the
npm lifecycle. Matches the pattern applied in solstone Makefile.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>