fix(gateway): remove DM thread session seeding to prevent cross-thread contamination (#7084)

The session store was copying the ENTIRE parent DM transcript into new
thread sessions. This caused unrelated conversations to bleed across
threads in Slack DMs.

The Slack adapter already handles thread context correctly via
_fetch_thread_context() (conversations.replies API), which fetches
only the actual thread messages. The session-level seeding was both
redundant and harmful.

No other platform (Telegram, Discord) uses DM threads, so the seeding
code path was only triggered by Slack — where it conflicted with the
adapter-level context.

Tests updated to assert thread isolation: all thread sessions start
empty, platform adapters are responsible for injecting thread context.

Salvage of PR #5868 (jarvisxyz). Reported by norbert on Discord.

gateway/session.py

@@ -770,41 +770,6 @@ class SessionStore:
             except Exception as e:
                 print(f"[gateway] Warning: Failed to create SQLite session: {e}")
 
-        # Seed new DM thread sessions with parent DM session history.
-        # When a bot reply creates a Slack thread and the user responds in it,
-        # the thread gets a new session (keyed by thread_ts).  Without seeding,
-        # the thread session starts with zero context — the user's original
-        # question and the bot's answer are invisible.  Fix: copy the parent
-        # DM session's transcript into the new thread session so context carries
-        # over while still keeping threads isolated from each other.
-        if (
-            source.chat_type == "dm"
-            and source.thread_id
-            and entry.created_at == entry.updated_at  # brand-new session
-            and not was_auto_reset
-        ):
-            parent_source = SessionSource(
-                platform=source.platform,
-                chat_id=source.chat_id,
-                chat_type="dm",
-                user_id=source.user_id,
-                # no thread_id — this is the parent DM session
-            )
-            parent_key = self._generate_session_key(parent_source)
-            with self._lock:
-                parent_entry = self._entries.get(parent_key)
-            if parent_entry and parent_entry.session_id != entry.session_id:
-                try:
-                    parent_history = self.load_transcript(parent_entry.session_id)
-                    if parent_history:
-                        self.rewrite_transcript(entry.session_id, parent_history)
-                        logger.info(
-                            "[Session] Seeded DM thread session %s with %d messages from parent %s",
-                            entry.session_id, len(parent_history), parent_entry.session_id,
-                        )
-                except Exception as e:
-                    logger.warning("[Session] Failed to seed thread session: %s", e)
-
         return entry
 
     def update_session(