core: remove redundant TurnContext and Prompt fields (#28638)

## Why

`TurnContext` had accumulated dead fields and cached projections of
values already owned by its per-turn `Config` or `ModelInfo`. Keeping
both copies made ownership unclear and allowed artificial split-brain
states, such as a compatibility hash differing from the model metadata
it came from.

`Prompt` similarly carried a write-only personality after personality
selection had already been materialized into its base instructions.

This makes the canonical owner explicit: configuration-backed values
come from `config`, model-derived values come from `model_info`, and
prompts contain only data consumed by request construction.

## What changed

- Remove the unused `ghost_snapshot`, `codex_self_exe`, and
`thread_source` fields.
- Remove duplicate `comp_hash`, `truncation_policy`, `features`,
`shell_environment_policy`, `codex_linux_sandbox_exe`, `compact_prompt`,
and `tool_mode` fields.
- Read those values directly from `TurnContext::config` or
`TurnContext::model_info` at their consumers.
- Remove the write-only `Prompt::personality` field and its constructor
assignments.
- Preserve review-turn inheritance of the parent turn's shell policy,
Linux sandbox executable, and compact prompt through the review config.

## Testing

- `cargo check -p codex-core --tests`

codex-rs/core/src/tasks/compact.rs

@@ -31,6 +31,7 @@ impl SessionTask for CompactTask {
         let session = session.clone_session();
         let _ = if crate::compact::should_use_remote_compact_task(ctx.provider.info()) {
             if ctx
+                .config
                 .features
                 .enabled(codex_features::Feature::RemoteCompactionV2)
             {
@@ -55,7 +56,12 @@ impl SessionTask for CompactTask {
                 /*manual*/ true,
             );
             let input = vec![UserInput::Text {
-                text: ctx.compact_prompt().to_string(),
+                text: ctx
+                    .config
+                    .compact_prompt
+                    .as_deref()
+                    .unwrap_or(crate::compact::SUMMARIZATION_PROMPT)
+                    .to_string(),
                 // Compaction prompt is synthesized; no UI element ranges to preserve.
                 text_elements: Vec::new(),
             }];