feat(FN-675): align session-files endpoint with diff endpoint

- Add git diff for session-files using baseCommitSha with double-dot syntax
- Add merge-base fallback to find common ancestor when baseCommitSha is missing
- Add HEAD~1 fallback when merge-base fails (e.g., no remote main)
- Add 10-second in-memory cache for session-files computation
- Fix scheduler to immediately schedule tasks created via dashboard
- Add task:moved to done handler for immediate scheduling on status change
- Add comprehensive unit tests for scheduler immediate scheduling
- Add unit tests for session-files caching and base ref resolution

packages/engine/src/scheduler.ts

@@ -94,6 +94,16 @@ export class Scheduler {
     private store: TaskStore,
     private options: SchedulerOptions = {},
   ) {
+    /**
+     * Event-driven scheduling: when a task is created, trigger a scheduling
+     * pass immediately instead of waiting for the next poll interval.
+     * This reduces latency from up to 15 seconds to near-instant.
+     */
+    this.store.on("task:created", () => {
+      schedulerLog.log("Task created — triggering scheduling");
+      this.schedule();
+    });
+
     /**
      * Immediate unpause resume: when `globalPause` transitions from `true`
      * to `false`, trigger a scheduling pass right away instead of waiting
@@ -152,6 +162,14 @@ export class Scheduler {
       if (task.sliceId && this.options.missionStore && to === "in-progress") {
         void this.handleMissionTaskStart(task.id, task.sliceId);
       }
+
+      // Event-driven scheduling: when a dependency completes (task moves to "done"),
+      // trigger scheduling immediately so waiting tasks can start without waiting
+      // for the next poll interval (up to 15 seconds).
+      if (to === "done") {
+        schedulerLog.log("Task completed — triggering scheduling");
+        this.schedule();
+      }
     });
 
     /**