html-skills-listen — server-mode setup for interactive artifacts

This skill is a system primitive for the html-skills plugin's interactive artifacts. It runs a bundled bash script that handles environment detection, idempotency, ephemeral-port startup, log parsing, and stale-session cleanup. After the script returns, you arm a Monitor on the receiver's stdout so each submit becomes a session notification, and return the URL to the parent skill.

Steps

Run the setup script:
```
bash scripts/listen.sh
```
The script is self-locating (resolves server.js next to itself via BASH_SOURCE), so it doesn't depend on $CLAUDE_PLUGIN_ROOT being set in your bash environment.

Output is KEY=VALUE lines on stdout. Always present: SID, LOG, MIDF, STATUS. When STATUS=STARTED or STATUS=ALREADY_RUNNING you also get URL. Capture LOG, MIDF, URL.
Branch on STATUS:
- STATUS=WEB — Claude Code web session. The sandbox can't reach the user's browser. Don't arm Monitor. Tell the parent skill (or the user, if invoked directly):