fix: persist compressed context to gateway session after mid-run compression

When context compression fires during run_conversation() in the gateway,
the compressed messages were silently lost on the next turn. Two bugs:

1. Agent-side: _flush_messages_to_session_db() calculated
   flush_from = max(len(conversation_history), _last_flushed_db_idx).
   After compression, _last_flushed_db_idx was correctly reset to 0,
   but conversation_history still had its original pre-compression
   length (e.g. 200). Since compressed messages are shorter (~30),
   messages[200:] was empty — nothing written to the new session's
   SQLite.

   Fix: Set conversation_history = None after each _compress_context()
   call so start_idx = 0 and all compressed messages are flushed.

2. Gateway-side: history_offset was always len(agent_history) — the
   original pre-compression length. After compression shortened the
   message list, agent_messages[200:] was empty, causing the gateway
   to fall back to writing only a user/assistant pair, losing the
   compressed summary and tail context.

   Fix: Detect session splits (agent.session_id != original) and set
   history_offset = 0 so all compressed messages are written to JSONL.

run_agent.py

@@ -6250,6 +6250,12 @@ class AIAgent:
                     )
                     if len(messages) >= _orig_len:
                         break  # Cannot compress further
+                    # Compression created a new session — clear the history
+                    # reference so _flush_messages_to_session_db writes ALL
+                    # compressed messages to the new session's SQLite, not
+                    # skipping them because conversation_history is still the
+                    # pre-compression length.
+                    conversation_history = None
                     # Re-estimate after compression
                     _preflight_tokens = estimate_request_tokens_rough(
                         messages,
@@ -7765,6 +7771,10 @@ class AIAgent:
                             approx_tokens=self.context_compressor.last_prompt_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 (see preflight compression comment).
+                        conversation_history = None
                     
                     # Save session log incrementally (so progress is visible even if interrupted)
                     self._session_messages = messages