feat(FN-3954): blocker staleness fanout and escalation for tasks

Merges FN-3954: adds a blocker fanout system where task staleness automatically escalates to upstream blocking tasks, with a new `blocker-fanout` core module, dashboard hooks wiring that surfaces escalation status on the board and executor status bar, and a new settings toggle to disable escalation.

Fusion-Task-Id: FN-3954

packages/core/src/settings-schema.ts

@@ -216,6 +216,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   specStalenessEnabled: false,
   specStalenessMaxAgeMs: 6 * 60 * 60 * 1000,
   taskStuckTimeoutMs: 600_000,
+  staleHighFanoutBlockerAgeThresholdMs: 2 * 60 * 60 * 1000,
   aiSessionTtlMs: 7 * 24 * 60 * 60 * 1000,
   aiSessionCleanupIntervalMs: 60 * 60 * 1000,
   autoUnpauseEnabled: true,