feat: final platform plugin parity — webhook delivery, platform hints, docs

Closes remaining functional gaps and adds documentation.

## Functional fixes

webhook.py: Cross-platform delivery now checks the plugin registry
  for unknown platform names instead of hardcoding 15 names in a tuple.
  Plugin platforms can receive webhook-routed deliveries.

prompt_builder: Platform hints (system prompt LLM guidance) now fall
  back to the plugin registry's platform_hint field. Plugin platforms
  can tell the LLM 'you're on IRC, no markdown.'

PlatformEntry: Added platform_hint field for LLM guidance injection.

IRC adapter: Added acquire_scoped_lock/release_scoped_lock in
  connect/disconnect to prevent two profiles from using the same IRC
  identity. Added platform_hint for IRC-specific LLM guidance.

Removed dead token-empty-warning extension for plugin platforms
  (plugin adapters handle their own env vars via check_fn).

## Documentation

website/docs/developer-guide/adding-platform-adapters.md:
  - Added 'Plugin Path (Recommended)' section with full code examples,
    PLUGIN.yaml template, config.yaml examples, and a table showing all
    18 integration points the plugin system handles automatically
  - Renamed built-in checklist to clarify it's for core contributors

gateway/platforms/ADDING_A_PLATFORM.md:
  - Added Plugin Path section pointing to the reference implementation
    and full docs guide
  - Clarified built-in path is for core contributors only

gateway/platforms/webhook.py

@@ -186,25 +186,21 @@ class WebhookAdapter(BasePlatformAdapter):
         if deliver_type == "github_comment":
             return await self._deliver_github_comment(content, delivery)
 
-        # Cross-platform delivery — any platform with a gateway adapter
-        if self.gateway_runner and deliver_type in (
-            "telegram",
-            "discord",
-            "slack",
-            "signal",
-            "sms",
-            "whatsapp",
-            "matrix",
-            "mattermost",
-            "homeassistant",
-            "email",
-            "dingtalk",
-            "feishu",
-            "wecom",
-            "wecom_callback",
-            "weixin",
-            "bluebubbles",
-        ):
+        # Cross-platform delivery — any platform with a gateway adapter.
+        # Check both built-in names and plugin-registered platforms.
+        _BUILTIN_DELIVER_PLATFORMS = {
+            "telegram", "discord", "slack", "signal", "sms", "whatsapp",
+            "matrix", "mattermost", "homeassistant", "email", "dingtalk",
+            "feishu", "wecom", "wecom_callback", "weixin", "bluebubbles",
+        }
+        _is_known_platform = deliver_type in _BUILTIN_DELIVER_PLATFORMS
+        if not _is_known_platform:
+            try:
+                from gateway.platform_registry import platform_registry
+                _is_known_platform = platform_registry.is_registered(deliver_type)
+            except Exception:
+                pass
+        if self.gateway_runner and _is_known_platform:
             return await self._deliver_cross_platform(
                 deliver_type, content, delivery
             )