feat(atlas): add background task session tracking with retry scheduling

- Add background-launch-session-tracking.ts to persist delegated sessions
- Add task-context.ts for task context resolution utilities
- Modify idle-event.ts to schedule retries when background tasks are running
- Update tool-execute-after.ts to integrate session tracking
- Add comprehensive tests for background task retry behavior

🤖 Generated with assistance of OhMyOpenCode

src/hooks/atlas/task-context.ts

@@ -0,0 +1,45 @@
+import { readCurrentTopLevelTask } from "../../features/boulder-state"
+import type { PendingTaskRef, TrackedTopLevelTaskRef } from "./types"
+
+export function resolvePreferredSessionId(currentSessionId?: string, trackedSessionId?: string): string {
+  return currentSessionId ?? trackedSessionId ?? "<session_id>"
+}
+
+export function resolveTaskContext(
+  pendingTaskRef: PendingTaskRef | undefined,
+  planPath: string,
+): {
+  currentTask: TrackedTopLevelTaskRef | null
+  shouldSkipTaskSessionUpdate: boolean
+  shouldIgnoreCurrentSessionId: boolean
+} {
+  if (!pendingTaskRef) {
+    return {
+      currentTask: readCurrentTopLevelTask(planPath),
+      shouldSkipTaskSessionUpdate: false,
+      shouldIgnoreCurrentSessionId: false,
+    }
+  }
+
+  if (pendingTaskRef.kind === "track") {
+    return {
+      currentTask: pendingTaskRef.task,
+      shouldSkipTaskSessionUpdate: false,
+      shouldIgnoreCurrentSessionId: false,
+    }
+  }
+
+  if (pendingTaskRef.reason === "explicit_resume") {
+    return {
+      currentTask: readCurrentTopLevelTask(planPath),
+      shouldSkipTaskSessionUpdate: true,
+      shouldIgnoreCurrentSessionId: true,
+    }
+  }
+
+  return {
+    currentTask: pendingTaskRef.task,
+    shouldSkipTaskSessionUpdate: true,
+    shouldIgnoreCurrentSessionId: true,
+  }
+}