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/llm-replay/tests/llm-replay.spec.ts

@@ -130,11 +130,11 @@ describe('deriveReplayScript', () => {
   })
 
   it('throws on a group that lacks a terminal finish chunk (a thrown stream)', () => {
-    // A thrown stream(): prefix chunks logged, then error/turn/end, NO finish.
+    // A thrown stream(): prefix chunks logged, then turn/end (error reason), NO finish.
     const events: SessionEvent[] = [
       chunkEvent(1, 1, 1, { type: 'block-start', index: 0, blockType: 'text' }),
       chunkEvent(2, 1, 1, { type: 'text-delta', index: 0, text: 'par' }),
-      { type: 'turn/end', seq: 3, time: 0, data: { turn: 1, reason: { kind: 'error', message: 'x' } } },
+      { type: 'turn/end', seq: 3, time: 0, data: { turn: 1, reason: { kind: 'error', step: 1, message: 'x' } } },
     ]
     expect(() => deriveReplayScript(events)).toThrow(/without a finish chunk.*replay\.override\.json/s)
   })