Agents window: consolidate redundant progress spinners

## Feedback

> there are like 5 spinners on my screen rn LOL

**Source**: #ws-modernize-ui — Justin Chen on 2026-07-15
**Reactions**: 🌀 ×1, loading-spinner ×4

Thread discussion:

> **Benjamin Simmonds**: 1. The one above the input will be removed soon. 2. The inline subagents should have some indication showing they are still in progress, but maybe instead of a spinner we use some more lightweight animation? 3. I'm not sure what we should do about the chat spinner — I feel like it's not needed when there is only one chat or if it's the active chat.
>
> **Justin Chen**: I think we talked a bit about making inline subagent shimmer instead.
>
> **Joanna Oikawa**: FWIW Justin's been trying to move us away from the spinner specifically, instead using shimmer for progress indications with text, or the braille spinner specifically for a session as a whole.

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## Context

As part of the Modern UI work, multiple progress spinners can appear on screen simultaneously (above the chat input, inline subagents, and the per-chat spinner), which reads as visually noisy and makes it unclear what is actually in progress. The team wants to consolidate progress affordances — favoring a lightweight shimmer for in-progress text/inline subagents and reserving a single (braille) spinner for a session as a whole — and to suppress redundant indicators (e.g. the chat spinner when there is only one chat or it's the active chat).

## Acceptance criteria

- [ ] Audit the concurrent progress indicators in the chat/agents surface and document each one's trigger.
- [ ] Replace inline subagent spinners with a lightweight shimmer/in-progress indication.
- [ ] Suppress the per-chat spinner when there is only a single chat or for the active chat, so only one meaningful progress indicator is visible at a time.