fix: clear conversation_history after mid-loop compression to prevent empty sessions (#7001)

After mid-loop compression (triggered by 413, context_overflow, or Anthropic
long-context tier errors), _compress_context() creates a new session in SQLite
and resets _last_flushed_db_idx=0. However, conversation_history was not cleared,
so _flush_messages_to_session_db() computed:

    flush_from = max(len(conversation_history=200), _last_flushed_db_idx=0) = 200
    messages[200:] → empty (compressed messages < 200)

This resulted in zero messages being written to the new session's SQLite store.
On resume, the user would see 'Session found but has no messages.'

The preflight compression path (line 7311) already had the fix:
    conversation_history = None

This commit adds the same clearing to the three mid-loop compression sites:
- Anthropic long-context tier overflow
- HTTP 413 payload too large
- Generic context_overflow error

Reported by Aaryan (Nous community).

run_agent.py

@@ -8344,6 +8344,10 @@ class AIAgent:
                                 approx_tokens=approx_tokens,
                                 task_id=effective_task_id,
                             )
+                            # Compression created a new session — clear history
+                            # so _flush_messages_to_session_db writes compressed
+                            # messages to the new session, not skipping them.
+                            conversation_history = None
                             if len(messages) < original_len or old_ctx > _reduced_ctx:
                                 self._emit_status(
                                     f"🗜️ Context reduced to {_reduced_ctx:,} tokens "
@@ -8401,6 +8405,10 @@ class AIAgent:
                             messages, system_message, approx_tokens=approx_tokens,
                             task_id=effective_task_id,
                         )
+                        # Compression created a new session — clear history
+                        # so _flush_messages_to_session_db writes compressed
+                        # messages to the new session, not skipping them.
+                        conversation_history = None
 
                         if len(messages) < original_len:
                             self._emit_status(f"🗜️ Compressed {original_len} → {len(messages)} messages, retrying...")
@@ -8519,6 +8527,10 @@ class AIAgent:
                             messages, system_message, approx_tokens=approx_tokens,
                             task_id=effective_task_id,
                         )
+                        # Compression created a new session — clear history
+                        # so _flush_messages_to_session_db writes compressed
+                        # messages to the new session, not skipping them.
+                        conversation_history = None
 
                         if len(messages) < original_len or new_ctx and new_ctx < old_ctx:
                             if len(messages) < original_len: