Function shared_agent_dir 

fn shared_agent_dir(shared_dir: &Path, agent_id: &str) -> PathBuf

Directory holding one file per channel currently registering agent_id in the host-global shared registry.

Deliberately does NOT go through agents_dir() (which appends an agents segment for the per-channel registry, where the caller passes a bare data dir) — shared_dir here is already registry::resolve_shared_reactive_dir()’s output (.../shared/agents/reactive), so entries land at .../shared/agents/reactive/<agent>/<channel>.json as documented, not double-nested under an extra agents/ (reagent P2 on #2350 — harmless since read/write agreed internally, but contradicted the documented layout).