refactor(agent-loop): drop the steering/message session event

Steer, inject, and followup now land as durable user/message events on the
session surface; the steering/message event type and its ConversationNode
kind are removed from the client projection. Update tests, docs, generated
catalogs, and agent notes to match, and align the steering e2e fixture and
prompt inventory assertions with the durable user/message landing.

docs/core-data-structures/session.md

@@ -33,10 +33,11 @@ interface SessionEventMap {
   'turn/start': { turn: number }
   /**
    * Closes turn `turn` after `step`, the last entered step (`0` when none),
-   * with the {@link TurnEndReason} that ended it. The loop awaits
-   * `session/flush` after an ordinary turn ends before claiming the next queued
-   * item. Success commits the turn; rejection is reported live and does not
-   * prevent later work.
+   * with the {@link TurnEndReason} that ended it. The loop does not await a
+   * flush at turn boundaries: `dsh-session-checkpoint-policy` owns the
+   * per-request durability checkpoint, and consumers that read storage after
+   * `whenIdle()` flush themselves. Success commits the turn; rejection is
+   * reported live and does not prevent later work.
    */
   'turn/end': { turn: number; step: number; reason: TurnEndReason }
   /** Opens step `step` of turn `turn` — one model call plus the tool executions it requested. */
@@ -84,8 +85,6 @@ interface SessionEventMap {
     error?: { name: string; code: string }
     meta?: JsonValue
   }
-  /** Steering content injected between steps of a running turn. */
-  'steering/message': { turn: number; message: UserMessage }
   /** Whole-list snapshot; latest write wins on replay. Log-only UI state; never derived history. */
   'todo/write': { todos: TodoItem[] }
   /**
@@ -202,7 +201,7 @@ A proper discriminated union over `type` (not independent `type`/`data` unions),
  *
  * The {@link sourceEventSeqs} and {@link surfaceOp} fields are conditional:
  * they only exist on {@link SurfaceEventType} variants (`user/message`,
- * `assistant/message`, `tool/result`, `steering/message`).
+ * `assistant/message`, `tool/result`).
  * Non-surface events (boundary markers, chunks, usage, errors) never carry
  * surface metadata — the compiler enforces this at `Session.append()`
  * call sites.
@@ -236,7 +235,7 @@ For `assistant/message`, a present `sourceEventSeqs: []` is a complete known-emp
 
 ## Surface types
 
-The four message-producing types (`SurfaceEventType` — `user/message`, `assistant/message`, `tool/result`, `steering/message`) carry surface metadata declaring how they join the ordered derived surface. See the [session surface Agent Note](../../.agents/notes/implemented/architecture/2026-06-18-session-surface.md).
+The three message-producing types (`SurfaceEventType` — `user/message`, `assistant/message`, `tool/result`) carry surface metadata declaring how they join the ordered derived surface. See the [session surface Agent Note](../../.agents/notes/implemented/architecture/2026-06-18-session-surface.md).
 
 ### `SurfaceEventType` — the message-producing subset of event types
 
@@ -250,7 +249,6 @@ type SurfaceEventType =
   | 'user/message'
   | 'assistant/message'
   | 'tool/result'
-  | 'steering/message'
 ```
 
 ### `SurfaceOp` — how an event entered the surface
@@ -260,7 +258,7 @@ type SurfaceEventType =
  * How a session event entered the ordered surface. Only valid on
  * {@link SurfaceEventType} events.
  *
- * - `'append'`: added to the tail — normal path for user/assistant/tool/steering
+ * - `'append'`: added to the tail — normal path for user/assistant/tool
  *   messages.
  * - `{ op: 'replace', start, end }`: replaces surface nodes from `start`
  *   (inclusive) through `end` (inclusive) with this node. Both must exist as
@@ -498,7 +496,6 @@ declare class Session {
 - `assistant/message` → an assistant message with the event's provider/model provenance and optional adapter-private replay state. Raw `assistant/chunk` events are replay/UI data and are **skipped** in derivation (the assembled message is authoritative). An **empty-content** `assistant/message` is also skipped — a max-tokens step cut off with no content still records an `assistant/message` to host its usage/provenance, but a content-less assistant turn must not enter the provider transcript.
 - `tool/result` → a user message carrying a `tool-result` block.
 - `user/message` (injected context, i.e. non-`user` source) → a user-role message carrying its `content` verbatim at its chronological position; provenance and domain data live in its typed source.
-- `steering/message` → a user-role message carrying exact `content` at its chronological position; an optional envelope remains log-only display metadata.
 
 Everything else (`turn/*`, `step/*`, plugin-owned `llm/retry`) is structural and does not project into a message. Token accounting reads per-step `assistant/chunk { type: 'usage' }` records and treats `assistant/message.usage` as the committed-step fallback when no usage chunk exists; failed model-request attempts have no assistant message, so their usage chunk is the durable accounting record. Because this unreleased format intentionally has no compatibility promise, seed/load validation rejects request headers without provider+model and assistant messages without provider/model provenance instead of guessing a route for historical data.