feat(gateway): notify users when session auto-resets (#2519)

When a session expires (daily schedule or idle timeout) and is
automatically reset, send a notification to the user explaining
what happened:

  ◐ Session automatically reset (inactive for 24h).
    Conversation history cleared.
  Use /resume to browse and restore a previous session.
  Adjust reset timing in config.yaml under session_reset.

Notifications are suppressed when:
- The expired session had no activity (no tokens used)
- The platform is excluded (api_server, webhook by default)
- notify: false in config

Changes:
- session.py: _should_reset() returns reason string ('idle'/'daily')
  instead of bool; SessionEntry gains auto_reset_reason and
  reset_had_activity fields; old entry's total_tokens checked
- config.py: SessionResetPolicy gains notify (bool, default: true)
  and notify_exclude_platforms (default: api_server, webhook)
- run.py: sends notification via adapter.send() before processing
  the user's message, with activity + platform checks
- 13 new tests

Config (config.yaml):

  session_reset:
    notify: true
    notify_exclude_platforms: [api_server, webhook]

gateway/session.py

@@ -355,6 +355,8 @@ class SessionEntry:
     # 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
+    auto_reset_reason: Optional[str] = None  # "idle" or "daily"
+    reset_had_activity: bool = False  # whether the expired session had any messages
     
     def to_dict(self) -> Dict[str, Any]:
         result = {
@@ -573,16 +575,19 @@ class SessionStore:
 
         return False
 
-    def _should_reset(self, entry: SessionEntry, source: SessionSource) -> bool:
+    def _should_reset(self, entry: SessionEntry, source: SessionSource) -> Optional[str]:
         """
         Check if a session should be reset based on policy.
         
+        Returns the reset reason ("idle" or "daily") if a reset is needed,
+        or None if the session is still valid.
+        
         Sessions with active background processes are never reset.
         """
         if self._has_active_processes_fn:
             session_key = self._generate_session_key(source)
             if self._has_active_processes_fn(session_key):
-                return False
+                return None
 
         policy = self.config.get_reset_policy(
             platform=source.platform,
@@ -590,14 +595,14 @@ class SessionStore:
         )
         
         if policy.mode == "none":
-            return False
+            return None
         
         now = datetime.now()
         
         if policy.mode in ("idle", "both"):
             idle_deadline = entry.updated_at + timedelta(minutes=policy.idle_minutes)
             if now > idle_deadline:
-                return True
+                return "idle"
         
         if policy.mode in ("daily", "both"):
             today_reset = now.replace(
@@ -610,9 +615,9 @@ class SessionStore:
                 today_reset -= timedelta(days=1)
             
             if entry.updated_at < today_reset:
-                return True
+                return "daily"
         
-        return False
+        return None
     
     def has_any_sessions(self) -> bool:
         """Check if any sessions have ever been created (across all platforms).
@@ -654,7 +659,8 @@ class SessionStore:
         if session_key in self._entries and not force_new:
             entry = self._entries[session_key]
             
-            if not self._should_reset(entry, source):
+            reset_reason = self._should_reset(entry, source)
+            if not reset_reason:
                 entry.updated_at = now
                 self._save()
                 return entry
@@ -663,6 +669,9 @@ class SessionStore:
                 # should have already flushed memories proactively; discard
                 # the marker so it doesn't accumulate.
                 was_auto_reset = True
+                auto_reset_reason = reset_reason
+                # Track whether the expired session had any real conversation
+                reset_had_activity = entry.total_tokens > 0
                 self._pre_flushed_sessions.discard(entry.session_id)
                 if self._db:
                     try:
@@ -671,6 +680,8 @@ class SessionStore:
                         logger.debug("Session DB operation failed: %s", e)
         else:
             was_auto_reset = False
+            auto_reset_reason = None
+            reset_had_activity = False
         
         # Create new session
         session_id = f"{now.strftime('%Y%m%d_%H%M%S')}_{uuid.uuid4().hex[:8]}"
@@ -685,6 +696,8 @@ class SessionStore:
             platform=source.platform,
             chat_type=source.chat_type,
             was_auto_reset=was_auto_reset,
+            auto_reset_reason=auto_reset_reason,
+            reset_had_activity=reset_had_activity,
         )
         
         self._entries[session_key] = entry