When memory is enabled, the first conversation with a legacy shared agent
creates a per-user agent directory containing only memory.json (no
config.yaml). On the second turn, resolve_agent_dir() returned this
incomplete directory, causing load_agent_config() to fail with
"Agent config not found".
Require config.yaml to exist alongside the directory for both the
per-user and legacy paths, so that memory-only directories fall
through correctly. This aligns resolve_agent_dir with the existing
config.yaml check in list_custom_agents.
Refs: https://github.com/bytedance/deer-flow/issues/3390
b3c2cc42cf