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.

docs/rfc/README.md

@@ -51,7 +51,6 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 |---|---|
 | [Unify the agent id and the session id](proposed/simplification/2026-06-20-unify-agent-and-session-id.md) | 2026-06-20 |
 | [Stop mirroring durable boundaries as agent events](proposed/simplification/2026-06-20-remove-agent-boundary-mirror-events.md) | 2026-06-20 |
-| [Fold trace-only session facts into load-bearing events](proposed/simplification/2026-06-20-collapse-trace-only-session-events.md) | 2026-06-20 |
 
 ### Architecture
 
@@ -95,6 +94,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 | [Drop the unconsumed `llm/adapter-change` event](implemented/simplification/2026-06-20-drop-unconsumed-llm-adapter-change-event.md) | 2026-06-20 |
 | [Prune dead methods from the persistence and bash seams](implemented/simplification/2026-06-20-prune-dead-seam-methods.md) | 2026-06-20 |
 | [Keep one public stop primitive](implemented/simplification/2026-06-20-public-agent-stop-surface.md) | 2026-06-20 |
+| [Fold trace-only session facts into load-bearing events](implemented/simplification/2026-06-20-collapse-trace-only-session-events.md) | 2026-06-20 |
 
 ### Architecture