proxy status --json, for launchers that share a proxy

Written by an AI agent (Claude) on behalf of @jasonkuhrt.

A launcher that shares one proxy across projects has to decide whether the proxy already running is the one it needs: its port, HTTPS, TLDs, LAN or unprivileged mode. We're writing one for Alchemy (alchemy-run/alchemy#2889). Today the only source is the state directory's files (proxy.pid, proxy.port, proxy.tls, proxy.tlds, proxy.lan, proxy.unprivileged), whose names are internal.

Proposal: a read-only portless proxy status, with --json for scripts:

$ portless proxy status --json
{"running":true,"pid":1841,"port":443,"tls":true,"tlds":["localhost"],"lan":false,"unprivileged":false,"stateDir":"/Users/me/.portless"}

#399 asks for the bind mode in the same output.