#6631: fix: configure npm global prefix for non-root user in Docker

## Problem The Dockerfile switches to the non-root `node` user for security hardening, but npm's default global prefix (`/usr/local`) is not writable by this user. This causes `EACCES: permission denied` errors when installing skills via ClawdHub or any `npm install -g` command. ``` Error: EACCES: permission denied, mkdir '/usr/local/lib/node_modules/clawdhub' ``` ## Fix Add two lines after `USER node` to configure npm to use a user-writable directory: ```dockerfile RUN npm config set prefix /home/node/.npm-global ENV PATH="/home/node/.npm-global/bin:${PATH}" ``` This is the standard Node.js recommended fix for [resolving EACCES permissions errors](https://docs.npmjs.com/resolving-eacces-permissions-errors-when-installing-packages-globally). ## Scope Dockerfile only — 2 lines added. No other changes. Fixes #6620 Related: #4130 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the runtime Docker image to support global npm installs while running as the non-root `node` user. After switching to `USER node`, it creates a user-writable global prefix directory under `/home/node/.npm-global`, sets npm’s global prefix to that path, and adds the corresponding `bin` directory to `PATH`, preventing `EACCES` errors for `npm install -g` (e.g., when installing skills via ClawHub). <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is isolated to the Dockerfile and follows standard npm guidance; it creates the prefix directory before setting it and only affects runtime global install behavior for the non-root `node` user. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->