core: remove redundant TurnContext and Prompt fields

[pakrym-oai]

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