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.

hermes_cli/plugins_cmd.py

@@ -630,10 +630,9 @@ def _plugin_exists(name: str) -> bool:
             manifest = _read_manifest(child)
             if manifest.get("name") == name:
                 return True
-    # Bundled: <repo>/plugins/<name>/
-    from pathlib import Path as _P
-    import hermes_cli
-    repo_plugins = _P(hermes_cli.__file__).resolve().parent.parent / "plugins"
+    # Bundled: <repo>/plugins/<name>/ (or HERMES_BUNDLED_PLUGINS on Nix).
+    from hermes_cli.plugins import get_bundled_plugins_dir
+    repo_plugins = get_bundled_plugins_dir()
     if repo_plugins.is_dir():
         candidate = repo_plugins / name
         if candidate.is_dir() and (
@@ -660,8 +659,8 @@ def _discover_all_plugins() -> list:
     seen: dict = {}  # name -> (name, version, description, source, path)
 
     # Bundled (<repo>/plugins/<name>/), excluding memory/ and context_engine/
-    import hermes_cli
-    repo_plugins = Path(hermes_cli.__file__).resolve().parent.parent / "plugins"
+    from hermes_cli.plugins import get_bundled_plugins_dir
+    repo_plugins = get_bundled_plugins_dir()
     for base, source in ((repo_plugins, "bundled"), (_plugins_dir(), "user")):
         if not base.is_dir():
             continue