Extract codex-instructions crate (#15744)

## Summary
- extract instruction fragment and user-instruction types into
codex-instructions
- update codex-core to consume the new crate

## Testing
- CI

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/instructions/src/lib.rs

@@ -0,0 +1,11 @@
+//! User and skill instruction payloads and contextual user fragment markers for Codex prompts.
+
+mod fragment;
+mod user_instructions;
+
+pub use fragment::AGENTS_MD_FRAGMENT;
+pub use fragment::ContextualUserFragmentDefinition;
+pub use fragment::SKILL_FRAGMENT;
+pub use user_instructions::SkillInstructions;
+pub use user_instructions::USER_INSTRUCTIONS_PREFIX;
+pub use user_instructions::UserInstructions;