fix(FN-2384): schedule triage and todo tasks by priority-aware FIFO

- Sort eligible todo tasks in Scheduler by priority first, then createdAt/id for stable FIFO ordering within each tier
- Sort eligible triage tasks using the same priority-aware ordering while preserving existing pause/status/recovery gating
- Add regression tests for scheduler and triage ordering, including blocked/paused/recovery-gated edge cases
- Update architecture docs to describe priority-first task dispatch behavior

packages/engine/src/scheduler.ts

@@ -1,4 +1,13 @@
-import { getCurrentRepo, resolveDependencyOrder, type TaskStore, type Task, type MissionStore, type MissionFeature, type PrInfo } from "@fusion/core";
+import {
+  getCurrentRepo,
+  resolveDependencyOrder,
+  sortTasksByPriorityThenAgeAndId,
+  type TaskStore,
+  type Task,
+  type MissionStore,
+  type MissionFeature,
+  type PrInfo,
+} from "@fusion/core";
 import { existsSync } from "node:fs";
 import { readFile } from "node:fs/promises";
 import { join } from "node:path";
@@ -563,6 +572,8 @@ export class Scheduler {
 
       if (todo.length === 0) return;
 
+      todo = sortTasksByPriorityThenAgeAndId(todo);
+
       /**
        * Pre-compute file scopes for all currently active tasks (in-progress
        * AND in-review with unmerged worktrees) so that todo tasks are never