fix: wire PII redaction + token empty warnings for plugin platforms

PII redaction: build_session_context_prompt() now checks the plugin
registry's pii_safe flag in addition to the hardcoded _PII_SAFE_PLATFORMS
frozenset. Plugin platforms that set pii_safe=True (e.g. phone-based
messaging bridges) get their user IDs redacted before LLM context.

Token empty warnings: the empty-token diagnostic at config load now
checks the plugin registry's required_env when a platform isn't in the
hardcoded _token_env_names dict. Catches 'enabled but empty' for
plugin platforms too.

gateway/config.py

@@ -891,6 +891,15 @@ def _validate_gateway_config(config: "GatewayConfig") -> None:
         if not pconfig.enabled:
             continue
         env_name = _token_env_names.get(platform)
+        if not env_name:
+            # Check plugin registry for required_env
+            try:
+                from gateway.platform_registry import platform_registry
+                entry = platform_registry.get(platform.value)
+                if entry and entry.required_env:
+                    env_name = entry.required_env[0]  # primary env var
+            except Exception:
+                pass
         if env_name and pconfig.token is not None and not pconfig.token.strip():
             logger.warning(
                 "%s is enabled but %s is empty. "

gateway/session.py

@@ -246,8 +246,18 @@ def build_session_context_prompt(
     Platforms like Discord are excluded because mentions need real IDs.
     Routing still uses the original values (they stay in SessionSource).
     """
-    # Only apply redaction on platforms where IDs aren't needed for mentions
+    # Only apply redaction on platforms where IDs aren't needed for mentions.
-    redact_pii = redact_pii and context.source.platform in _PII_SAFE_PLATFORMS
+    # Check both the hardcoded set (builtins) and the plugin registry.
+    _is_pii_safe = context.source.platform in _PII_SAFE_PLATFORMS
+    if not _is_pii_safe:
+        try:
+            from gateway.platform_registry import platform_registry
+            entry = platform_registry.get(context.source.platform.value)
+            if entry and entry.pii_safe:
+                _is_pii_safe = True
+        except Exception:
+            pass
+    redact_pii = redact_pii and _is_pii_safe
     lines = [
         "## Current Session Context",
         "",