simplify(session): fold trace-only usage/error events into load-bearing events

The session event vocabulary carried two standalone trace-only events that
were not load-bearing as separate records. Fold their facts into nearby
load-bearing events and delete the standalone variants.

- Token usage now rides on `assistant/message` as an optional `usage` field —
  the assembled model output and its accounting travel together. The loop folds
  `assembler.usage` onto the append instead of emitting a separate `usage`
  event.
- The max-tokens path is the no-data-loss host: a step cut off with usage but
  EMPTY content (e.g. only a dropped tool call) previously emitted a standalone
  `usage`; it now records an empty-content `assistant/message { content: [],
  usage }`. `deriveMessages()` skips empty-content assistant messages, so the
  usage host never injects a spurious content-less assistant turn into the
  provider transcript. A step with neither content nor usage appends nothing.
- An operational error's step number now rides on `turn/end.reason` for
  `kind: 'error'` (`{ kind: 'error', step, message, code? }`) — the durable
  turn outcome ACP and resume already consume. `failTurn` sets the reason
  directly (no separate session `error` event). `agent/error` + logging are
  unchanged for live diagnostics.
- No format-version bump: pre-release, no persisted data, so per the format
  policy there is nothing to migrate or reject (the RFC's "refresh the format
  version" criterion over-reached). `version` stays 1.
- ACP fixtures + goldens re-recorded (keyless replay): dropped standalone
  usage/error lines, usage folded onto assistant/message, error step on
  turn/end.reason.

RFC moved proposed -> implemented with an implementation note recording the two
scope refinements.

packages/core/session/src/index.ts

@@ -160,7 +160,10 @@ export class Session {
    *
    * - `user/message` → user message
    * - `assistant/message` → assistant message (chunks are skipped — they are
-   *   replay/UI data; the assembled message is authoritative for history)
+   *   replay/UI data; the assembled message is authoritative for history). An
+   *   EMPTY-content assistant/message is skipped: a max-tokens step cut off with
+   *   no content still records an assistant/message to host its `usage`, but a
+   *   content-less assistant turn must not enter the provider transcript.
    * - `tool/result` → user message carrying a tool-result block
    * - `context/message` / `steering/message` → tagged synthetic user messages
    *   at their chronological position
@@ -186,6 +189,10 @@ export class Session {
           break
         }
         case 'assistant/message': {
+          // Skip an empty-content assistant/message: it exists only to host a
+          // max-tokens step's usage and must not inject a content-less assistant
+          // turn into the provider transcript.
+          if (event.data.content.length === 0) break
           messages.push({ role: 'assistant', content: structuredClone(event.data.content) })
           break
         }

packages/core/session/src/types.ts

@@ -88,7 +88,13 @@ export type TurnTrigger = TurnTriggerMap[keyof TurnTriggerMap]
 export interface TurnEndReasonMap {
   completed: { kind: 'completed' }
   aborted: { kind: 'aborted'; reason?: string }
-  error: { kind: 'error'; message: string; code?: string }
+  /**
+   * The turn failed: a step threw or the model reported a failure. `step` is the
+   * step number the failure occurred on (the operational error's location — the
+   * single durable record of an in-turn failure; live diagnostics also fire via
+   * `agent/error`). `code` is the error's code when one was attached.
+   */
+  error: { kind: 'error'; step: number; message: string; code?: string }
   disposed: { kind: 'disposed' }
   'max-tokens': { kind: 'max-tokens' }
   /**
@@ -140,14 +146,17 @@ export interface SessionEventMap {
   'context/message': { content: ContentBlock[]; source: MessageSource }
   /** Raw stream chunk — token-level replay fidelity. */
   'assistant/chunk': { turn: number; step: number; chunk: StreamChunk }
-  /** Assembled assistant message for one step (derived history uses this). */
-  'assistant/message': { turn: number; step: number; content: ContentBlock[] }
+  /**
+   * Assembled assistant message for one step (derived history uses this).
+   * Carries the step's `usage` when the adapter reported token accounting, so
+   * the model output and its accounting travel together (there is no separate
+   * usage record). `usage` is absent when the adapter reported none.
+   */
+  'assistant/message': { turn: number; step: number; content: ContentBlock[]; usage?: TokenUsage }
   'tool/call': { turn: number; step: number; callId: CallId; name: string; arguments: string }
   'tool/result': { turn: number; step: number; callId: CallId; content: ContentBlock[]; isError: boolean; error?: { name: string; code: string } }
   /** Steering content injected between steps of a running turn. */
   'steering/message': { turn: number; content: ContentBlock[]; source: MessageSource }
-  'usage': { turn: number; step: number; usage: TokenUsage }
-  'error': { turn: number; step: number; message: string; code?: string }
 }
 
 export type SessionEventType = keyof SessionEventMap