fix(gateway): re-inject topic-bound skill after /new or /reset

reset_session() creates a fresh SessionEntry with created_at == updated_at,
but get_or_create_session() bumps updated_at on the next inbound message,
causing _is_new_session in _handle_message_with_agent to evaluate False.
The topic/channel skill auto-load gate (group_topics, channel_skill_bindings)
silently skips the first message after a manual reset.

Add an is_fresh_reset flag on SessionEntry, set by reset_session() and
consumed once by the message handler. Kept distinct from was_auto_reset
because that flag also drives a 'session expired due to inactivity'
user-facing notice and a context-note prepend — both wrong for an
explicit /new or /reset.

Persisted through to_dict/from_dict so the flag survives gateway
restart between /reset and the next message.

Fixes #6508

Co-authored-by: warabe1122 <45554392+warabe1122@users.noreply.github.com>
Co-authored-by: willy-scr <187001140+willy-scr@users.noreply.github.com>

gateway/run.py

@@ -5300,7 +5300,12 @@ class GatewayRunner:
         _is_new_session = (
             session_entry.created_at == session_entry.updated_at
             or getattr(session_entry, "was_auto_reset", False)
+            or getattr(session_entry, "is_fresh_reset", False)
         )
+        # Consume the is_fresh_reset flag immediately so it doesn't leak
+        # onto subsequent messages in the same session (issue #6508).
+        if getattr(session_entry, "is_fresh_reset", False):
+            session_entry.is_fresh_reset = False
         if _is_new_session:
             await self.hooks.emit("session:start", {
                 "platform": source.platform.value if source.platform else "",