feat(team): share leader memory with team members (read-only)

Team members dispatched via team tasks can now read the leader's memory
files (MEMORY.md, memory/*.md) as a fallback when they have no own
memory. Memory writes are blocked for members — only the leader can
save memory files. This follows the existing team_workspace context
propagation pattern: dispatch metadata → RunRequest → context → tools.

Affected tools: read_file (interceptor), memory_get, memory_search,
list_files (interceptor). All include leader fallback with per-user →
global scope cascade.

cmd/gateway_consumer_handlers.go

@@ -287,6 +287,7 @@ func handleTeammateMessage(
 		Stream:          false,
 		TeamTaskID:      msg.Metadata["team_task_id"],
 		TeamWorkspace:   msg.Metadata["team_workspace"],
+		LeaderAgentID:   msg.Metadata["leader_agent_id"],
 		WorkspaceChatID: origChatID,
 		TeamID:          msg.Metadata["team_id"],
 		LinkedTraceID:   linkedTraceID,