feat: shared thread sessions by default — multi-user thread support (#5391)

Threads (Telegram forum topics, Discord threads, Slack threads) now default
to shared sessions where all participants see the same conversation. This is
the expected UX for threaded conversations where multiple users @mention the
bot and interact collaboratively.

Changes:
- build_session_key(): when thread_id is present, user_id is no longer
  appended to the session key (threads are shared by default)
- New config: thread_sessions_per_user (default: false) — opt-in to restore
  per-user isolation in threads if needed
- Sender attribution: messages in shared threads are prefixed with
  [sender name] so the agent can tell participants apart
- System prompt: shared threads show 'Multi-user thread' note instead of
  a per-turn User line (avoids busting prompt cache)
- Wired through all callers: gateway/run.py, base.py, telegram.py, feishu.py
- Regular group messages (no thread) remain per-user isolated (unchanged)
- DM threads are unaffected (they have their own keying logic)

Closes community request from demontut_ re: thread-based shared sessions.

gateway/config.py

@@ -246,6 +246,7 @@ class GatewayConfig:
 
     # Session isolation in shared chats
     group_sessions_per_user: bool = True  # Isolate group/channel sessions per participant when user IDs are available
+    thread_sessions_per_user: bool = False  # When False (default), threads are shared across all participants
 
     # Unauthorized DM policy
     unauthorized_dm_behavior: str = "pair"  # "pair" or "ignore"
@@ -333,6 +334,7 @@ class GatewayConfig:
             "always_log_local": self.always_log_local,
             "stt_enabled": self.stt_enabled,
             "group_sessions_per_user": self.group_sessions_per_user,
+            "thread_sessions_per_user": self.thread_sessions_per_user,
             "unauthorized_dm_behavior": self.unauthorized_dm_behavior,
             "streaming": self.streaming.to_dict(),
         }
@@ -376,6 +378,7 @@ class GatewayConfig:
             stt_enabled = data.get("stt", {}).get("enabled") if isinstance(data.get("stt"), dict) else None
 
         group_sessions_per_user = data.get("group_sessions_per_user")
+        thread_sessions_per_user = data.get("thread_sessions_per_user")
         unauthorized_dm_behavior = _normalize_unauthorized_dm_behavior(
             data.get("unauthorized_dm_behavior"),
             "pair",
@@ -392,6 +395,7 @@ class GatewayConfig:
             always_log_local=data.get("always_log_local", True),
             stt_enabled=_coerce_bool(stt_enabled, True),
             group_sessions_per_user=_coerce_bool(group_sessions_per_user, True),
+            thread_sessions_per_user=_coerce_bool(thread_sessions_per_user, False),
             unauthorized_dm_behavior=unauthorized_dm_behavior,
             streaming=StreamingConfig.from_dict(data.get("streaming", {})),
         )
@@ -467,6 +471,9 @@ def load_gateway_config() -> GatewayConfig:
             if "group_sessions_per_user" in yaml_cfg:
                 gw_data["group_sessions_per_user"] = yaml_cfg["group_sessions_per_user"]
 
+            if "thread_sessions_per_user" in yaml_cfg:
+                gw_data["thread_sessions_per_user"] = yaml_cfg["thread_sessions_per_user"]
+
             streaming_cfg = yaml_cfg.get("streaming")
             if isinstance(streaming_cfg, dict):
                 gw_data["streaming"] = streaming_cfg