[BUG] Opening a second VS Code window breaks subtask completion: the parent task never resumes #1926

Summary

Opening a second VS Code window while a subtask is running in another window silently corrupts the delegation link between the parent task and that subtask. The new window's TaskHistoryStore startup reconciliation treats the running child as a crash orphan and rewrites both records: child active → interrupted, parent delegated → active with awaitingChildId cleared. When the subtask later calls attempt_completion, its delegation guard (parentHistory.awaitingChildId === task.taskId in src/core/tools/AttemptCompletionTool.ts) fails against the mangled history and takes the "parent already detached" path. The subtask shows as completed, and the parent never resumes.

Reproduction

1. In one VS Code window, run a task that delegates a subtask (parent delegated, child active).
2. While the subtask is running, open a second VS Code window. Both windows share the extension's single globalStorage/tasks directory.
3. Let the subtask reach attempt_completion and approve finishing the subtask.

Expected: the parent resumes with the subtask's result.
Observed: the subtask completes; the parent never resumes.

The trigger is the new window's startup, not user activity — a hands-free session only escapes it because it rarely coincides with a window launch.

Consequences

• The subtask's result never reaches the parent and the parent's continuation never runs; to the user the parent silently stops while the subtask shows completed.
• The persisted history contradicts reality (parent not awaiting, child interrupted, both tasks in fact live), which misroutes follow-up operations: ClineProvider.reopenParentFromDelegation() aborts with "parent is no longer delegated to child", and the child survives only as a history-replay resume whose parent link is already gone.
• Every window start that coincides with an in-flight delegation applies this rewrite — the repair's crash assumption is false for all live peers sharing the task store.

Root cause

TaskHistoryStore.initialize() runs reconcileDelegationState(), whose repair case "parent delegated, child active → child interrupted, parent active" rests on the stated assumption that "an active child persisted across startup cannot have a live task session behind it" (repairActiveDelegation()). With one extension host that is sound; with several VS Code windows sharing one task store it is not — window B's recovery pass mutates window A's running tasks.

Where to look

• src/core/task-persistence/TaskHistoryStore.ts — initialize(), reconcileDelegationState() / reconcileDelegationStateCore(), repairActiveDelegation()
• src/core/tools/AttemptCompletionTool.ts — the awaitingChildId delegation guard
• src/core/webview/ClineProvider.ts — reopenParentFromDelegation()'s stale-parent guard

Observed on a main build (v3.86.0, 2026-10-04).