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.

docs/core-data-structures/session.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write
-session.md: d789ffcabb5cb0c744e265b61e322831c1d8a04f
-session.zh.md: f4f102861db7403520e9f38cb56613e430718cbe
+session.md: 1a627bb316f9b348c6d30d1fb7ed9d575cc73c7d
+session.zh.md: 4acfb5fafc46bed45132a890bfcc456131707dd5

docs/core-data-structures/session.md

@@ -159,7 +159,7 @@ The unit of the `todo/write` event's whole-list snapshot. Deliberately minimal
 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'
 }
 ```

docs/core-data-structures/session.zh.md

@@ -159,7 +159,7 @@ interface OutOfBandSessionEventMap {}
 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'
 }
 ```