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)

docs/README.md

@@ -18,6 +18,11 @@ top-to-bottom once, then use the numbered docs as a lookup while implementing.
 
 ---
 
+## User-facing guides
+
+- [`install.md`](install.md) — **install the gateway + connect the app** (non-technical walkthrough, all options, TLS, push).
+- [`setup.md`](setup.md) — moved; pointer to `install.md`.
+
 ## Reading order
 
 | # | File | When to read |