fix: refresh updater panels from update and activity events

[atomantic]

Updater panels now read once and refresh from update/activity events instead of recurring 4-second and 15-second API polls. Reconnect and tab re-show reconcile missed changes, event bursts coalesce, and mutation responses remain ahead of stale reads. Restart health probes are unchanged.

The scheduler emits a bounded, payload-free invalidation after runtime/config changes, including cooldown skips, and watches external Git ref changes without watching the self-refreshing index. Working-tree-only edits reconcile through activity, scheduler ticks, and tab re-show.

Validation:

• Rendered updater tests cover event-driven changes, no periodic fetches, recovery, listener cleanup, and save/read races.
• Scheduler tests cover coalesced configuration/runtime/ref invalidation and cheap cooldown handling.
• Configured tool-free Codex reviewer: no findings.
• Pregate passed (lint, guards, related server/client tests); CI will verify build and boot smoke.
• Fixed an exposed Brain Threads test race by awaiting hydrated content instead of only the drawer shell.

Closes #8551
Part of #8530.