docs(session): scope the todo/write JSDoc to the event's own contract

Codex Phase 1 review: the event JSDoc described Phase 2 consumers (the
todo_write tool, stdio printing, ACP plan mapping) as current state, and put an
@mode tag on a SessionEventMap member. @mode is for first-class Cordis
`interface Events` entries the catalog generator reads — this event rides the
existing session/event emit and has no catalog row, so the tag was wrong.

Trim the JSDoc to the event's own contract (snapshot data shape,
last-write-wins, not-a-surface-event) and drop @mode; phrase TodoItem in terms
of its own purpose rather than a not-yet-present tool.

docs/core-data-structures/session.md

@@ -36,20 +36,17 @@ interface SessionEventMap {
   /** Steering content injected between steps of a running turn. */
   'steering/message': { turn: number; content: ContentBlock[]; source: MessageSource }
   /**
-   * The agent's whole todo list, replaced wholesale on each write
-   * (last-write-wins on replay — the current list is the last `todo/write`).
-   * Written by the `todo_write` tool via
-   * `agent.session.append('todo/write', { todos })`.
+   * The agent's whole todo list, carried as a full snapshot and replaced
+   * wholesale on each write — the current list is the most recent `todo/write`
+   * (last-write-wins on replay, no fold). Appended by an owning agent via
+   * `session.append('todo/write', { todos })`.
    *
    * NOT a {@link SurfaceEventType}: it produces no LLM message and never reaches
-   * `deriveMessages()` — it is durable, replayable UI state. The full snapshot
-   * travels each time, so a resume re-derives the current list from the last
-   * event with no fold. UIs render off `session/event`: the stdio UI prints the
-   * list; the ACP bridge maps it to a `plan` sessionUpdate. This is a
-   * `SessionEventMap` member (it rides the existing `session/event` emit), not a
-   * first-class `interface Events` notification, so the cordis catalog gains no
-   * row for it.
-   * @mode emit
+   * `deriveMessages()`, so it carries no `surfaceOp` and stays off the surface —
+   * it is durable, replayable UI state, distinct from the conversation history.
+   * It is a `SessionEventMap` member riding the existing `session/event` emit,
+   * not a first-class Cordis `interface Events` notification, so it has no
+   * cordis-catalog row.
    */
   'todo/write': { todos: TodoItem[] }
 }
@@ -57,7 +54,7 @@ interface SessionEventMap {
 
 ### `TodoItem` — one todo-list entry
 
-The unit of the `todo_write` tool's whole-list state. Deliberately minimal — a `content` line and a three-state `status` (no id, priority, or `activeForm`): the list is replaced wholesale on every write, so entries need no stable identity, and the status triple is exactly the ACP `PlanEntryStatus`, so the ACP bridge maps a todo list to a `plan` update 1:1 (synthesizing the priority ACP additionally requires).
+The unit of the `todo/write` event's whole-list snapshot. Deliberately minimal — a `content` line and a three-state `status` (no id, priority, or `activeForm`): the list is replaced wholesale on every write, so entries need no stable identity, and the status triple is exactly the ACP `PlanEntryStatus`, so a UI bridge can map a todo list onto an ACP `plan` 1:1 (synthesizing the priority ACP additionally requires).
 
 ```ts type-equiv
 export interface TodoItem {

packages/core/session/src/types.ts

@@ -150,14 +150,14 @@ export interface TurnEndReasonMap {
 export type TurnEndReason = TurnEndReasonMap[keyof TurnEndReasonMap]
 
 /**
- * One entry in an agent's todo list — the unit of the `todo_write` tool's
- * whole-list state (the `todo/write` {@link SessionEventMap} event).
+ * One entry in an agent's todo list — the unit of the `todo/write`
+ * {@link SessionEventMap} event's whole-list snapshot.
  *
  * Deliberately minimal: a human-readable `content` line and a three-state
  * `status`. No id, priority, or `activeForm` — the list is replaced wholesale
  * on every write (last-write-wins), so entries need no stable identity, and the
- * status triple is exactly the ACP `PlanEntryStatus` (so the ACP bridge maps a
- * todo list to a `plan` update 1:1, synthesizing the priority ACP additionally
+ * status triple is exactly the ACP `PlanEntryStatus`, so a UI bridge can map a
+ * todo list onto an ACP `plan` 1:1 (synthesizing the priority ACP additionally
  * requires).
  */
 export interface TodoItem {
@@ -213,20 +213,17 @@ export interface SessionEventMap {
   /** Steering content injected between steps of a running turn. */
   'steering/message': { turn: number; content: ContentBlock[]; source: MessageSource }
   /**
-   * The agent's whole todo list, replaced wholesale on each write
-   * (last-write-wins on replay — the current list is the last `todo/write`).
-   * Written by the `todo_write` tool via
-   * `agent.session.append('todo/write', { todos })`.
+   * The agent's whole todo list, carried as a full snapshot and replaced
+   * wholesale on each write — the current list is the most recent `todo/write`
+   * (last-write-wins on replay, no fold). Appended by an owning agent via
+   * `session.append('todo/write', { todos })`.
    *
    * NOT a {@link SurfaceEventType}: it produces no LLM message and never reaches
-   * `deriveMessages()` — it is durable, replayable UI state. The full snapshot
-   * travels each time, so a resume re-derives the current list from the last
-   * event with no fold. UIs render off `session/event`: the stdio UI prints the
-   * list; the ACP bridge maps it to a `plan` sessionUpdate. This is a
-   * `SessionEventMap` member (it rides the existing `session/event` emit), not a
-   * first-class `interface Events` notification, so the cordis catalog gains no
-   * row for it.
-   * @mode emit
+   * `deriveMessages()`, so it carries no `surfaceOp` and stays off the surface —
+   * it is durable, replayable UI state, distinct from the conversation history.
+   * It is a `SessionEventMap` member riding the existing `session/event` emit,
+   * not a first-class Cordis `interface Events` notification, so it has no
+   * cordis-catalog row.
    */
   'todo/write': { todos: TodoItem[] }
 }