fix(bridge): RUNNING 세션 우선 선택 + IDLE 채널 자동 생성 제거

- extension: bestSession 선택에 2단계 비교 (RUNNING > IDLE, then modTime)
- extension: [SESSION-FILTER] 진단 로그 + [projectName] 로그 접두사
- bot: pending_approval_scanner의 IDLE 프로젝트 자동 채널 생성 제거
- known-issues: 2개 항목 추가 (IDLE 고착, 채널 증식)

bot.py

@@ -511,11 +511,8 @@ class GravityBot(commands.Bot):
             # Reload conv→project registrations each cycle
             self._load_registrations()
 
-            # Ensure channels exist for all registered projects
-            for project in set(self.conv_to_project.values()):
-                if project not in self.project_channels:
-                    await self._get_channel(project)
-                    logger.info(f"Auto-created channel for registered project: {project}")
+            # Channels are created on-demand when actual signals arrive
+            # (via _get_channel in snapshot scanner / approval sender)
 
             requests = self.bridge.get_pending_requests()
             for req in requests: