## Why
Currently, the analytics reducer omits `codex_turn_event` for internally
started subagent turns
- It uses `TurnState.connection_id` to select app-server client and
runtime metadata
- `turn/start` sets this field for client-started turns, while internal
subagent turns bypass that path
- Spawned child threads inherit the correct connection, but turn
emission does not use thread state
## What Changed
- Keeps explicit `TurnState.connection_id` authoritative for
client-started turns
- Falls back to the matching thread’s inherited connection when the turn
connection is absent
- Preserves completeness gates, event schema, and post-emission state
removal
- Extends subagent lifecycle test coverage
## Verification
- `just test -p codex-analytics` (71 tests passed)
- `just fix -p codex-analytics`
- `just fmt`
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