fix: isolate static subagent registries by runtime context (#2371)

Fixes #2328 

Fix static subagent declaration isolation for namespace-scoped
filesystems.

Previously, `SubagentsMiddleware` refreshed a shared subagent registry
for every call. An interleaved call from another user could overwrite
the declarations and manager used by an in-flight call, causing the
prompt or `agent_spawn` to use another user's subagents.