fix: handle None message content across codebase (fixes #276)

The OpenAI API returns content: null on assistant messages with tool
calls. msg.get('content', '') returns None when the key exists with
value None, causing TypeError on len(), string concatenation, and
.strip() in downstream code paths.

Fixed 4 locations that process conversation messages:
- agent/auxiliary_client.py:84 — None passed to API calls
- cli.py:1288 — crash on content[:200] and len(content)
- run_agent.py:3444 — crash on None.strip()
- honcho_integration/session.py:445 — 'None' rendered in transcript

13 other instances were verified safe (already protected, only process
user/tool messages, or use the safe pattern).

Pattern: msg.get('content', '') → msg.get('content') or ''

Fixes #276

cli.py

@@ -1285,7 +1285,7 @@ class HermesCLI:
         
         for i, msg in enumerate(self.conversation_history, 1):
             role = msg.get("role", "unknown")
-            content = msg.get("content", "")
+            content = msg.get("content") or ""
             
             if role == "user":
                 print(f"\n  [You #{i}]")