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/tests/properties.spec.ts

@@ -24,6 +24,7 @@ const textContentArb = fc.array(
 const messageEventArb: fc.Arbitrary<Appendable> = fc.oneof(
   textContentArb.map((content): Appendable => ({ type: 'user/message', data: { content, source: { kind: 'user' } } })),
   textContentArb.map((content): Appendable => ({ type: 'assistant/message', data: { turn: 1, step: 1, content } })),
+  textContentArb.map((content): Appendable => ({ type: 'assistant/message', data: { turn: 1, step: 1, content, usage: { inputTokens: 1, outputTokens: 1 } } })),
   fc.record({ id: fc.string({ minLength: 1 }), content: textContentArb, isError: fc.boolean() })
     .map((r): Appendable => ({ type: 'tool/result', data: { turn: 1, step: 1, callId: CallId(r.id), content: r.content, isError: r.isError } })),
 )
@@ -35,8 +36,6 @@ const nonMessageEventArb: fc.Arbitrary<Appendable> = fc.oneof(
   fc.constant<Appendable>({ type: 'step/start', data: { turn: 1, step: 1 } }),
   fc.constant<Appendable>({ type: 'step/end', data: { turn: 1, step: 1 } }),
   fc.string().map((text): Appendable => ({ type: 'assistant/chunk', data: { turn: 1, step: 1, chunk: { type: 'text-delta', index: 0, text } } })),
-  fc.constant<Appendable>({ type: 'usage', data: { turn: 1, step: 1, usage: { inputTokens: 1, outputTokens: 1 } } }),
-  fc.constant<Appendable>({ type: 'error', data: { turn: 1, step: 1, message: 'x' } }),
 )
 
 const anyEventArb = fc.oneof(messageEventArb, nonMessageEventArb)