main folded the plugin's contextvars-inheriting thread wrapper into
agent.memory_provider.spawn_context_thread. The registry this stack adds for
join_plugin_threads keeps a thin client.spawn_context_thread that calls the core
spawner and records the thread under its owner; the four spawn sites import it
from client. main's inherit-profile test builds a real provider now that
_spawn_write is an instance method (the owner is what gets joined).
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