## Why
`TurnContextItem` is the durable baseline used to reconstruct context
diffs across resume/fork. Most of the old persisted-only fields on it
are no longer read, so keeping them in rollout snapshots adds schema
surface and state that can drift without affecting reconstruction.
`summary` is the exception: older Codex versions require it to
deserialize `turn_context` records, so keep writing a default
compatibility value until that schema surface can be removed safely.
## What changed
- Removed the unused persisted fields from `TurnContextItem`: trace ids,
user/developer instructions, output schema, and truncation policy.
- Kept `summary` with a compatibility comment and made
`TurnContext::to_turn_context_item` write `ReasoningSummary::Auto`
instead of live turn state.
- Updated rollout/context reconstruction fixtures for the retained
summary field.
## Verification
- `cargo test -p codex-protocol --lib turn_context_item`
- `cargo test -p codex-rollout
resume_candidate_matches_cwd_reads_latest_turn_context`
- `cargo test -p codex-state turn_context`
- `cargo test -p codex-core --lib
new_default_turn_captures_current_span_trace_id`
- `cargo test -p codex-core --lib
record_initial_history_resumed_turn_context_after_compaction_reestablishes_reference_context_item`
- `cargo test -p codex-core --test all
emits_warning_when_resumed_model_differs`
- `git diff --check`
f2368b7de6