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/session.py

@@ -234,7 +234,7 @@ def build_session_context_prompt(
 ) -> str:
     """
     Build the dynamic system prompt section that tells the agent about its context.
-    
+
     This is injected into the system prompt so the agent knows:
     - Where messages are coming from
     - What platforms are connected
@@ -246,13 +246,23 @@ 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
-    redact_pii = redact_pii and context.source.platform in _PII_SAFE_PLATFORMS
+    # Only apply redaction on platforms where IDs aren't needed for mentions.
+    # 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",
         "",
     ]
-    
+
     # Source info
     platform_name = context.source.platform.value.title()
     if context.source.platform == Platform.LOCAL:
@@ -277,7 +287,7 @@ def build_session_context_prompt(
         else:
             desc = src.description
         lines.append(f"**Source:** {platform_name} ({desc})")
-    
+
     # Channel topic (if available - provides context about the channel's purpose)
     if context.source.chat_topic:
         lines.append(f"**Channel Topic:** {context.source.chat_topic}")
@@ -302,7 +312,7 @@ def build_session_context_prompt(
         if redact_pii:
             uid = _hash_sender_id(uid)
         lines.append(f"**User ID:** {uid}")
-    
+
     # Platform-specific behavioral notes
     if context.source.platform == Platform.SLACK:
         lines.append("")
@@ -368,9 +378,9 @@ def build_session_context_prompt(
     for p in context.connected_platforms:
         if p != Platform.LOCAL:
             platforms_list.append(f"{p.value}: Connected ✓")
-    
+
     lines.append(f"**Connected Platforms:** {', '.join(platforms_list)}")
-    
+
     # Home channels
     if context.home_channels:
         lines.append("")
@@ -378,11 +388,11 @@ def build_session_context_prompt(
         for platform, home in context.home_channels.items():
             hc_id = _hash_chat_id(home.chat_id) if redact_pii else home.chat_id
             lines.append(f"  - {platform.value}: {home.name} (ID: {hc_id})")
-    
+
     # Delivery options for scheduled tasks
     lines.append("")
     lines.append("**Delivery options for scheduled tasks:**")
-    
+
     from hermes_constants import display_hermes_home
 
     # Origin delivery
@@ -398,15 +408,15 @@ def build_session_context_prompt(
     lines.append(
         f"- `\"local\"` → Save to local files only ({display_hermes_home()}/cron/output/)"
     )
-    
+
     # Platform home channels
     for platform, home in context.home_channels.items():
         lines.append(f"- `\"{platform.value}\"` → Home channel ({home.name})")
-    
+
     # Note about explicit targeting
     lines.append("")
     lines.append("*For explicit targeting, use `\"platform:chat_id\"` format if the user provides a specific chat ID.*")
-    
+
     return "\n".join(lines)