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
         }