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/user_instructions_tests.rs

@@ -0,0 +1,72 @@
+use super::*;
+use codex_protocol::models::ContentItem;
+use codex_protocol::models::ResponseItem;
+use pretty_assertions::assert_eq;
+
+use crate::fragment::AGENTS_MD_FRAGMENT;
+use crate::fragment::SKILL_FRAGMENT;
+
+#[test]
+fn test_user_instructions() {
+    let user_instructions = UserInstructions {
+        directory: "test_directory".to_string(),
+        text: "test_text".to_string(),
+    };
+    let response_item: ResponseItem = user_instructions.into();
+
+    let ResponseItem::Message { role, content, .. } = response_item else {
+        panic!("expected ResponseItem::Message");
+    };
+
+    assert_eq!(role, "user");
+
+    let [ContentItem::InputText { text }] = content.as_slice() else {
+        panic!("expected one InputText content item");
+    };
+
+    assert_eq!(
+        text,
+        "# AGENTS.md instructions for test_directory\n\n<INSTRUCTIONS>\ntest_text\n</INSTRUCTIONS>",
+    );
+}
+
+#[test]
+fn test_is_user_instructions() {
+    assert!(AGENTS_MD_FRAGMENT.matches_text(
+        "# AGENTS.md instructions for test_directory\n\n<INSTRUCTIONS>\ntest_text\n</INSTRUCTIONS>"
+    ));
+    assert!(!AGENTS_MD_FRAGMENT.matches_text("test_text"));
+}
+
+#[test]
+fn test_skill_instructions() {
+    let skill_instructions = SkillInstructions {
+        name: "demo-skill".to_string(),
+        path: "skills/demo/SKILL.md".to_string(),
+        contents: "body".to_string(),
+    };
+    let response_item: ResponseItem = skill_instructions.into();
+
+    let ResponseItem::Message { role, content, .. } = response_item else {
+        panic!("expected ResponseItem::Message");
+    };
+
+    assert_eq!(role, "user");
+
+    let [ContentItem::InputText { text }] = content.as_slice() else {
+        panic!("expected one InputText content item");
+    };
+
+    assert_eq!(
+        text,
+        "<skill>\n<name>demo-skill</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>",
+    );
+}
+
+#[test]
+fn test_is_skill_instructions() {
+    assert!(SKILL_FRAGMENT.matches_text(
+        "<skill>\n<name>demo-skill</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>"
+    ));
+    assert!(!SKILL_FRAGMENT.matches_text("regular text"));
+}