feat(gateway): unify setup flows, load platforms dynamically from registry

Merge the two gateway setup paths (hermes setup gateway + hermes gateway
setup) to use a single _unified_platforms() list that merges built-in
_PLATFORMS with dynamically registered plugin entries from
platform_registry.

- Add setup_fn field to PlatformEntry for plugin setup flows
- _unified_platforms() merges built-ins with registry entries by key
- setup_gateway() now uses unified list instead of hardcoded
  _GATEWAY_PLATFORMS tuple list
- gateway_setup() uses same unified list, plugin entries appear
  alongside built-ins with no [plugin] suffix
- _platform_status() handles plugin platforms via registry check_fn
- Plugin platforms with setup_fn get called directly; plugins without
  get a generic env-var display fallback

IRC and other plugin platforms now appear automatically in the setup
menu when registered via platform_registry.register().

feat(gateway): surface disabled platform plugins in setup and auto-enable on select

Platform plugins under plugins/platforms/* (IRC, etc.) were gated behind
plugins.enabled, so `hermes gateway setup` wouldn't list them until the
user ran `hermes plugins enable <name>` first. Now the setup menu always
surfaces them as "plugin disabled — select to enable", and picking one
adds it to plugins.enabled before running its setup flow.

Along the way, unify the two gateway setup flows so `hermes setup gateway`
and `hermes gateway setup` both read from the same platform list (built-in
_PLATFORMS + platform_registry entries), dispatch through a single
_configure_platform() helper, and share _platform_status(). Deletes the
dead bespoke wrappers in setup.py (_setup_whatsapp, _setup_weixin,
_setup_email, etc.) that duplicated logic now covered by the registry
path or _setup_standard_platform.

Also:
- PlatformEntry gains a plugin_name field so the registry knows which
  plugin owns each entry (required for auto-enable).
- PluginContext.register_platform auto-stamps plugin_name from the
  manifest so plugins don't have to pass it explicitly.
- PluginManager now scans plugins/platforms/* as its own category root,
  one level below the bundled plugin scan.
- Fix IRC plugin discovery: rename PLUGIN.yaml → plugin.yaml (the
  scanner is case-sensitive) and add the missing __init__.py that
  _load_directory_module requires.

tests/hermes_cli/test_setup.py

@@ -163,12 +163,13 @@ def test_setup_gateway_skips_service_install_when_systemctl_missing(monkeypatch,
         "WEBHOOK_ENABLED": "",
     }
 
+    import hermes_cli.gateway as gateway_mod
+
     monkeypatch.setattr(setup_mod, "get_env_value", lambda key: env.get(key, ""))
+    monkeypatch.setattr(gateway_mod, "get_env_value", lambda key: env.get(key, ""))
     monkeypatch.setattr(setup_mod, "prompt_yes_no", lambda *args, **kwargs: False)
     monkeypatch.setattr("platform.system", lambda: "Linux")
 
-    import hermes_cli.gateway as gateway_mod
-
     monkeypatch.setattr(gateway_mod, "supports_systemd_services", lambda: False)
     monkeypatch.setattr(gateway_mod, "is_macos", lambda: False)
     monkeypatch.setattr(gateway_mod, "_is_service_installed", lambda: False)
@@ -201,12 +202,13 @@ def test_setup_gateway_in_container_shows_docker_guidance(monkeypatch, capsys):
         "WEBHOOK_ENABLED": "",
     }
 
+    import hermes_cli.gateway as gateway_mod
+
     monkeypatch.setattr(setup_mod, "get_env_value", lambda key: env.get(key, ""))
+    monkeypatch.setattr(gateway_mod, "get_env_value", lambda key: env.get(key, ""))
     monkeypatch.setattr(setup_mod, "prompt_yes_no", lambda *args, **kwargs: False)
     monkeypatch.setattr("platform.system", lambda: "Linux")
 
-    import hermes_cli.gateway as gateway_mod
-
     monkeypatch.setattr(gateway_mod, "supports_systemd_services", lambda: False)
     monkeypatch.setattr(gateway_mod, "is_macos", lambda: False)
     monkeypatch.setattr(gateway_mod, "_is_service_installed", lambda: False)