Desktop spawns managed agents with BUZZ_ACP_MULTIPLE_EVENT_HANDLING hard-coded to steer — no per-agent exposure

Start with MultipleEventHandling in crates/buzz-acp/src/config.rs and the managed-agent spawn path in desktop/src-tauri/src/managed_agents/runtime.rs. Trace how idle_timeout_seconds and max_turn_duration_seconds are stored and passed to the command, then determine where the per-agent setting belongs. Done means a selected event-handling mode is exposed at the requested scope and replaces the unconditional steer environment value.

Component

desktop (managed agents) / buzz-acp

Summary

The runner supports four mid-turn delivery modes (crates/buzz-acp/src/config.rs, MultipleEventHandling): queue, steer (default), interrupt, owner-interrupt. The CLI flag and BUZZ_ACP_MULTIPLE_EVENT_HANDLING work fine when spawning buzz-acp manually.

But managed agents spawned by the desktop app cannot use any of this: the spawn path hard-codes the env (desktop/src-tauri/src/managed_agents/runtime.rs at ef2aa1a):

command.env("BUZZ_ACP_MULTIPLE_EVENT_HANDLING", "steer");
command.env("BUZZ_ACP_DEDUP", "queue");

It is unconditional, while the neighboring envs shown it being written deliberately key off record fields:

if let Some(idle) = record.idle_timeout_seconds {
    command.env("BUZZ_ACP_IDLE_TIMEOUT", idle.to_string());
}
if let Some(max_dur) = record.max_turn_duration_seconds {
    command.env("BUZZ_ACP_MAX_TURN_DURATION", max_dur.to_string());
}

So idle_timeout_seconds / max_turn_duration_seconds are exposed per agent, but the event-handling mode is not — there is no UI setting and no agent-record field, even though the mechanism clearly exists.

Impact

Managed agents are always subject to steering semantics: a @mention arriving mid-turn cancels the in-flight turn and merges (see also #5839 for the channel-scoped variant of this). Users who want queue semantics — never cancel, deliver the new message after the current turn — have no way to choose it for a managed agent.

Request

Expose multiple_event_handling as a per-managed-agent setting (like idle timeout and max turn duration already are), or at minimum a workspace-level default, and pass the chosen value into the spawn env instead of the hard-coded steer.

Verified against

commit ef2aa1a (fresh checkout of main). Happy to link a PR if maintainers agree with the shape.