## What

The `agentFileChangeReport` for a turn now includes files changed by the
subagents that turn started. Fixes IJAI-3823.

## Why

`CodexEventHandler` kept `turn/diff/updated` only from the root thread.
Subagents report diffs under their own thread and turn ids, so a turn
where only a subagent edited files produced `paths: []` and Air showed
no "Since last turn" card.

## How

- Child-thread diffs are recorded per child thread and turn, attributed
to the root turn that was running (or the last one, if the subagent
outlives it). Recording happens before subagent routing, so diffs of
ended children or never-materialized spawns are kept.
- `getTurnDiff` appends the attributed child diffs to the root diff;
`isTurnDiffOversized` also accounts for oversized child diffs.

Not covered: clients without native subagent sessions (child events
never reach the handler), and subagent follow-up turns that run after
the prompt ends.