credits.grant_spent is a one-time "your monthly grant is used up, you're now on
top-up" heads-up, but it was sticky — it camped the TUI status bar until the grant
refilled, so a user with healthy top-up saw "Grant spent · $990 top-up left"
indefinitely. Treat it like the usage-band notice: flash once, then clear on the
next prompt (startMessage). Depletion stays sticky (you actually can't make
requests). The Python `active` latch keeps the key, so it won't re-fire next turn.
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