[idea] (IMPLEMENTED) Bring back animated agent status spinners as an opt-in config

[carterlasalle]

Honestly I was a bit sad to see the spinners go in 0.8.0. I get why they were removed, static marks mean no constant re-rendering while agents work, and #967 had people complaining about cursor flicker, so static as the default seems fair to me.

But I'd really like the option to turn them back on. I usually have a few agents running across workspaces and I keep glancing at the sidebar while I work in the terminal. With the spinner I can tell right away which workspace is still churning. The static yellow dot just says "busy somewhere", so I end up reading the rows to find what I'm looking for.

So my ask: bring the old animation back behind a flag. ui.agent_spinner = true, default off, with a toggle in Settings > indicators. Since it defaults to off, people who don't want it never see any of the extra rendering or the flicker risk.

I've got this working on a branch already (ui/agent-spinner-config), tested and all, so if you're open to the idea I'd open a PR. Happy to rename the flag or put the toggle somewhere else if you'd rather do it differently.

Would love to hear what you think.

[carterlasalle]

One more thing on why I care, since the flag name and defaults are easy to bikeshed:

The main case for me is running several agents at once. I glance at the sidebar from the terminal pane all the time, and with the spinner I can tell which workspace is still working without reading anything. With the static dot I get "busy somewhere" and that's it, so I end up scanning rows.

About #967: those complaints were about the default experience, where the animation meant constant re-renders and the flicker. Off by default means nobody gets any of that unless they opt in.

The branch is in decent shape. It's basically the pre-0.8 animation code gated behind the flag, mobile/headless rendering included, and the tests pass (only failures are pre-existing env stuff, unrelated).

@ogulcancelik if a default-off flag sounds reasonable, the branch is ready and I'd open the PR. Also fine with renaming it or moving the toggle in settings.