feat: inject crash context into next session's system prompt

When the agent crashes with an unhandled exception, record the error
(type + message) on the SessionEntry.  On the next message, inject a
[System note: ...] into the context prompt so the agent wakes up
knowing what failed and can self-diagnose instead of starting blind.

Three injection points:
- SessionEntry: new fields last_error_type / last_error_message /
  last_error_time (gateway/session.py)
- Exception handler: record crash on session_entry before returning
  error to user (gateway/run.py:~9107)
- Context prompt builder: prepend crash note and clear fields so
  the notice appears exactly once (gateway/run.py:~8216)

The agent sees: [System note: The previous session crashed with
ValueError: Model gpt-5.5 has a context window of 4,096 tokens...]

gateway/run.py

@@ -8216,6 +8216,28 @@ class GatewayRunner:
                 context_note = "[System note: The user's previous session expired due to inactivity. This is a fresh conversation with no prior context.]"
             context_prompt = context_note + "\n\n" + context_prompt
 
+        # If the previous agent turn crashed, prepend a crash-context notice
+        # so the agent knows what went wrong and can self-diagnose.  The
+        # error fields are cleared after this read so the notice appears
+        # exactly once (on the first message of the fresh session).
+        _last_err_type = getattr(session_entry, 'last_error_type', None)
+        if _last_err_type:
+            _last_err_msg = getattr(session_entry, 'last_error_message', '') or ''
+            _crash_note = (
+                f"[System note: The previous session crashed with "
+                f"{_last_err_type}: {_last_err_msg[:300]}. "
+                f"Diagnose and fix the root cause if possible — "
+                f"do NOT just retry the same thing.]"
+            )
+            context_prompt = _crash_note + "\n\n" + context_prompt
+            # Clear so the notice appears only once
+            try:
+                session_entry.last_error_type = None
+                session_entry.last_error_message = None
+                session_entry.last_error_time = None
+            except Exception:
+                pass
+
             # Send a user-facing notification explaining the reset, unless:
             # - notifications are disabled in config
             # - the platform is excluded (e.g. api_server, webhook)
@@ -9098,6 +9120,14 @@ class GatewayRunner:
             logger.exception("Agent error in session %s", session_key)
             error_type = type(e).__name__
             error_detail = str(e)[:300] if str(e) else "no details available"
+            # Record the crash on the session entry so the next session's
+            # agent can self-diagnose the failure instead of starting blind.
+            try:
+                session_entry.last_error_type = error_type
+                session_entry.last_error_message = error_detail
+                session_entry.last_error_time = time.time()
+            except Exception:
+                pass  # defensive — never let error-recording cause a secondary crash
             status_hint = ""
             status_code = getattr(e, "status_code", None)
             _hist_len = len(history) if 'history' in locals() else 0

gateway/session.py

@@ -453,6 +453,13 @@ class SessionEntry:
     # Last API-reported prompt tokens (for accurate compression pre-check)
     last_prompt_tokens: int = 0
     
+    # Set when the previous agent turn crashed with an unhandled exception.
+    # Consumed once by the message handler to inject a crash-context notice
+    # into the next session's system prompt so the agent can self-diagnose.
+    last_error_type: Optional[str] = None
+    last_error_message: Optional[str] = None
+    last_error_time: Optional[float] = None
+
     # Set when a session was created because the previous one expired;
     # consumed once by the message handler to inject a notice into context
     was_auto_reset: bool = False