feat(todo): allow several in_progress todos at once

Remove the single-in_progress cap from todo_write execute validation and
the durable-log invariant so a task list can mirror genuinely parallel
work (concurrent subagents, background commands). Update the tool
description to instruct marking every actively worked task in_progress,
refresh the tool catalog and keyless snapshot expected outputs, and
record the decision in a new Agent Note superseding the original cap.

packages/core/session/src/types.ts

@@ -146,7 +146,7 @@ export type TurnEndReason = TurnEndReasonMap[keyof TurnEndReasonMap]
 export interface TodoItem {
   /** What this task is — a short imperative line shown in the UI. */
   content: string
-  /** Lifecycle state. `in_progress` marks the single task being worked now. */
+  /** Lifecycle state. `in_progress` marks a task being worked now; parallel work may mark several. */
   status: 'pending' | 'in_progress' | 'completed'
 }