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/support/invariants/src/index.ts

@@ -192,8 +192,8 @@ function checkEvent(trace: SessionTrace, event: SessionEvent): void {
     // turn as its commit/replay boundary (the JSONL backend treats anything
     // after the last turn/end as a crash tail), so a bare event between turns is
     // silently dropped on reload. The loop records queued user messages after
-    // turn/start, an idle agent.inject() wraps its context/message in a one-shot
-    // turn, and usage/error are only appended inside an open turn. A `default`
+    // turn/start, and an idle agent.inject() wraps its context/message in a
+    // one-shot turn. A `default`
     // (not an enumerated list) is deliberate: SessionEventMap is
     // merge-extensible, so a PLUGIN-added event type appended while idle must
     // also fail here rather than fall through and be dropped on resume.