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/ui/acp/src/index.ts

@@ -775,7 +775,7 @@ function validateMcpServers(params: { mcpServers?: unknown[] }): void {
  * generic fallback (title = tool name, raw args as input) when no registry is
  * available (e.g. pure translator tests).
  *
- * Other event types (turn/step boundaries, context/message, usage, …) produce
+ * Other event types (turn/step boundaries, context/message, …) produce
  * no client update.
  */
 export function streamSessionEventUpdate(
@@ -873,7 +873,7 @@ export function streamSessionEventUpdate(
       })
       return
     }
-    // turn/step boundaries, context/message, steering, usage, error,
+    // turn/step boundaries, context/message, steering,
     // assistant/message — no direct ACP client update.
     default:
       return

packages/ui/acp/tests/codec.spec.ts

@@ -16,7 +16,7 @@ describe('turnEndToStopReason', () => {
     expect(turnEndToStopReason({ kind: 'max-tokens' })).toBe('max_tokens')
     expect(turnEndToStopReason({ kind: 'aborted', reason: 'x' })).toBe('cancelled')
     expect(turnEndToStopReason({ kind: 'disposed' })).toBe('cancelled')
-    expect(turnEndToStopReason({ kind: 'error', message: 'boom' })).toBe('end_turn')
+    expect(turnEndToStopReason({ kind: 'error', step: 1, message: 'boom' })).toBe('end_turn')
   })
 
   it('falls back to end_turn for an unknown (merge-extensible) future kind', () => {

packages/ui/acp/tests/stream-update.spec.ts

@@ -116,7 +116,7 @@ describe('streamSessionEventUpdate', () => {
   it('produces no update for boundary/other event types', () => {
     expect(updatesFor(evt('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } }))).toEqual([])
     expect(updatesFor(evt('turn/end', { turn: 1, reason: { kind: 'completed' } }))).toEqual([])
-    expect(updatesFor(evt('usage', { turn: 1, step: 1, usage: { inputTokens: 1, outputTokens: 1 } }))).toEqual([])
+    expect(updatesFor(evt('step/start', { turn: 1, step: 1 }))).toEqual([])
   })
 })