docs+plugin: HTTP-only transport cleanup, install guide, review fixes

- docs/install.md: new end-to-end guide for non-technical users
  (gateway install, app install, LAN/TLS/remote connection, push,
  options, troubleshooting); docs/setup.md now points to it
- README: new 'Install the gateway' section; pairing section updated
  for HTTP transport (8791, QR scan on Android)
- rename IRIS_WS_HOST -> IRIS_HTTP_HOST (clean rename, no compat
  fallback); drop dead DEFAULT_PORT=8790
- setup.py: advertise https:// in the printed/QR server URL when
  IRIS_HTTP_CERT is set
- ws_probe.py/e2e.py: default --url http://127.0.0.1:8791, env
  IRIS_WS_URL -> IRIS_HTTP_URL, honor explicit port + https scheme
- plugin.yaml: IRIS_HTTP_* env names, description no longer says
  'WebSocket server'
- docs 03/09/12/19: fix stale WS-era refs (ws_server.py cites,
  8790 smoke test, WSS->HTTPS, 'HTTP fallback' reframed as the
  only transport)
- AGENTS.md: symlink name android -> iris (matches actual install)
- test: adapter reads IRIS_HTTP_HOST/CERT/KEY from env; legacy
  IRIS_WS_* names are not consulted (95/95 pass)

gateway-plugin/defaults.py

@@ -1,8 +1,7 @@
 """Platform defaults (config.yaml ``extra`` / env fallbacks)."""
 
 DEFAULT_HOST = "127.0.0.1"
-DEFAULT_PORT = 8790
-DEFAULT_HTTP_PORT = 8791  # docs/19: HTTP fallback leg
+DEFAULT_HTTP_PORT = 8791  # docs/19: HTTP is the only transport
 DEFAULT_HOME_CHANNEL = "default"
 DEFAULT_HOME_CHANNEL_NAME = "Default"
 DEFAULT_PUSH_BACKEND = "ntfy"