perf(agent-control): reduce proposal countdown churn

[beruro]

Summary

• replace frame-rate countdown updates with a single one-second timer
• pause countdown work while the window is hidden and recalculate from the absolute expiry time on foreground
• own timer/listener teardown in a small scheduler with lifecycle regression tests

Performance guard

• active/visible: at most one timer and one update per second
• hidden: no timer remains armed
• visible again: one immediate recalculation, then one timer
• unmount/reopen: timer and listener are removed; every scheduler instance owns isolated state
• failure/retry: not applicable; this path performs no I/O

Verdict: PASS. The change removes approximately 60 React updates per second from the five-minute proposal window and retains bounded O(1) state.

Verification

• vitest run src/scaffold/GlobalSpotlight/palettes/AgentControlPalette/countdownScheduler.test.ts (3 tests)
• tsc --noEmit
• focused ESLint
• Prettier check
• git diff --check

[beruro]

Restacked onto #512. This PR now contains only the extracted countdown scheduler and its regression coverage (3 files). Merge #512 first, then retarget to develop for CI.