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/run.py

@@ -1694,12 +1694,54 @@ class GatewayRunner:
         # If the previous session expired and was auto-reset, prepend a notice
         # so the agent knows this is a fresh conversation (not an intentional /reset).
         if getattr(session_entry, 'was_auto_reset', False):
-            context_prompt = (
-                "[System note: The user's previous session expired due to inactivity. "
-                "This is a fresh conversation with no prior context.]\n\n"
-                + context_prompt
-            )
+            reset_reason = getattr(session_entry, 'auto_reset_reason', None) or 'idle'
+            if reset_reason == "daily":
+                context_note = "[System note: The user's session was automatically reset by the daily schedule. This is a fresh conversation with no prior context.]"
+            else:
+                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
+
+            # Send a user-facing notification explaining the reset, unless:
+            # - notifications are disabled in config
+            # - the platform is excluded (e.g. api_server, webhook)
+            # - the expired session had no activity (nothing was cleared)
+            try:
+                policy = self.session_store.config.get_reset_policy(
+                    platform=source.platform,
+                    session_type=getattr(source, 'chat_type', 'dm'),
+                )
+                platform_name = source.platform.value if source.platform else ""
+                had_activity = getattr(session_entry, 'reset_had_activity', False)
+                should_notify = (
+                    policy.notify
+                    and had_activity
+                    and platform_name not in policy.notify_exclude_platforms
+                )
+                if should_notify:
+                    adapter = self.adapters.get(source.platform)
+                    if adapter:
+                        if reset_reason == "daily":
+                            reason_text = f"daily schedule at {policy.at_hour}:00"
+                        else:
+                            hours = policy.idle_minutes // 60
+                            mins = policy.idle_minutes % 60
+                            duration = f"{hours}h" if not mins else f"{hours}h {mins}m" if hours else f"{mins}m"
+                            reason_text = f"inactive for {duration}"
+                        notice = (
+                            f"◐ Session automatically reset ({reason_text}). "
+                            f"Conversation history cleared.\n"
+                            f"Use /resume to browse and restore a previous session.\n"
+                            f"Adjust reset timing in config.yaml under session_reset."
+                        )
+                        await adapter.send(
+                            source.chat_id, notice,
+                            metadata=getattr(event, 'metadata', None),
+                        )
+            except Exception as e:
+                logger.debug("Auto-reset notification failed (non-fatal): %s", e)
+
             session_entry.was_auto_reset = False
+            session_entry.auto_reset_reason = None
         
         # Load conversation history from transcript
         history = self.session_store.load_transcript(session_entry.session_id)