[idea] Choose which agent events raise toasts: needs-attention only, without "finished"

[darkguy2008]

idea / problem

I run many agents in parallel and only want to be interrupted when one is blocked and needs a decision from me. Agents finishing is expected and frequent; I check on them through the sidebar when I'm ready. Today every background completion raises a "finished" toast, so the "needs attention" toasts that actually matter get lost among them.

What exists today:

• ui.toast.delivery = "off" silences everything, including needs-attention, and also disables herdr notification show, which my own keybinding scripts use for feedback.
• [ui.sound] has separate done_path and request_path, but toasts have no equivalent per-kind control.

Related but different: #2529 asks to silence specific orchestrated agents. This asks to silence one kind of event for all agents.

requested change

A config option to choose which lifecycle events raise toasts, so that "needs attention" (blocked) can stay on while "finished" is turned off. For example, a list such as ui.toast.events = ["needs_attention"] that defaults to both, or a boolean per kind. Not prescribing the design.

Ideally the same choice would apply to terminal and system delivery, and leave herdr notification show unaffected.