Spinner verb cycling is too fast — make interval configurable

The cycling verbs (Thinking…, Mustering…, Pondering…, etc.) currently rotate roughly every second. At that rate they read like a strobe / blinking light in peripheral vision, which is fatiguing during longer waits (e.g. while waiting for a tool-heavy response).

A slower rotation — every 10–20 seconds — would keep the flavor without the distraction. Ideally exposed as a setting (e.g. spinnerVerbInterval: 15) so users can dial it themselves. Settings like spinnerVerbs already exist; this would be a small additive change to the same surface.

Context: VS Code extension. The existing spinnerVerbs / spinnerTipsEnabled / prefersReducedMotion settings don't appear to affect the extension's panel (only the terminal CLI), but that's a separate issue. This request is purely about cycling speed.