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.

cron/scheduler.py

@@ -341,28 +341,6 @@ def _deliver_result(job: dict, content: str, adapters=None, loop=None) -> Option
     from tools.send_message_tool import _send_to_platform
     from gateway.config import load_gateway_config, Platform
 
-    # Accept any platform name — built-in names resolve to their enum
-    # member, plugin platform names create dynamic members via _missing_().
-    try:
-        platform = Platform(platform_name.lower())
-    except (ValueError, KeyError):
-        msg = f"unknown platform '{platform_name}'"
-        logger.warning("Job '%s': %s", job["id"], msg)
-        return msg
-
-    try:
-        config = load_gateway_config()
-    except Exception as e:
-        msg = f"failed to load gateway config: {e}"
-        logger.error("Job '%s': %s", job["id"], msg)
-        return msg
-
-    pconfig = config.platforms.get(platform)
-    if not pconfig or not pconfig.enabled:
-        msg = f"platform '{platform_name}' not configured/enabled"
-        logger.warning("Job '%s': %s", job["id"], msg)
-        return msg
-
     # Optionally wrap the content with a header/footer so the user knows this
     # is a cron delivery.  Wrapping is on by default; set cron.wrap_response: false
     # in config.yaml for clean output.
@@ -419,13 +397,23 @@ def _deliver_result(job: dict, content: str, adapters=None, loop=None) -> Option
                 job["id"], platform_name, chat_id, thread_id,
             )
 
-        platform = platform_map.get(platform_name.lower())
-        if not platform:
+        # Built-in names resolve to their enum member; plugin platform names
+        # create dynamic members via Platform._missing_().
+        try:
+            platform = Platform(platform_name.lower())
+        except (ValueError, KeyError):
             msg = f"unknown platform '{platform_name}'"
             logger.warning("Job '%s': %s", job["id"], msg)
             delivery_errors.append(msg)
             continue
 
+        pconfig = config.platforms.get(platform)
+        if not pconfig or not pconfig.enabled:
+            msg = f"platform '{platform_name}' not configured/enabled"
+            logger.warning("Job '%s': %s", job["id"], msg)
+            delivery_errors.append(msg)
+            continue
+
         # Prefer the live adapter when the gateway is running — this supports E2EE
         # rooms (e.g. Matrix) where the standalone HTTP path cannot encrypt.
         runtime_adapter = (adapters or {}).get(platform)