[observability] Add turn lifecycle observations\n\nIntroduce shared turn.started and turn.ended observations, including terminal\nstatus, timing, and optional provider token usage on the terminal event.\n\nProject those observations into the existing analytics reducer state and add a\nside-by-side conformance test showing the observation path produces the same\ncodex_turn_event payload as the legacy inputs for the migrated fields.\n\nUpdate the design doc so turn token accounting is modeled as part of the\nterminal turn event rather than a separate canonical event.

docs/observability-event-stream-design.md

@@ -270,7 +270,7 @@ Initial workflow coverage should be chosen by conformance need:
 | Workflow | Canonical examples | Primary consumers |
 | --- | --- | --- |
 | Session/thread | `session.config_resolved`, `thread.started`, `thread.ended` | analytics, OTEL, rollout |
-| Turn lifecycle | `turn.requested`, `turn.started`, `turn.ended`, `turn.token_usage_observed` | analytics, OTEL, rollout |
+| Turn lifecycle | `turn.requested`, `turn.started`, `turn.ended` | analytics, OTEL, rollout |
 | Turn timing | `turn.first_token_observed`, `turn.first_message_observed` | OTEL metrics, rollout |
 | Model I/O | `inference.started`, `inference.sse_event_observed`, `inference.completed`, `inference.failed` | OTEL, rollout |
 | Tools | `tool_call.started`, `tool_call.approval_resolved`, `tool_call.ended` | OTEL, rollout |
@@ -292,7 +292,7 @@ output schemas.
 | Observations | Analytics output |
 | --- | --- |
 | `thread.started` | `codex_thread_initialized` |
-| turn config/lifecycle/token observations | `codex_turn_event` |
+| `turn.started`, `turn.ended` with token usage, and future config observations | `codex_turn_event` |
 | `turn.steer_resolved` | `codex_turn_steer_event` |
 | compaction lifecycle observations | `codex_compaction_event` |
 | skill/app/plugin/guardian observations | existing feature events |