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.rs

@@ -0,0 +1,56 @@
+use serde::Deserialize;
+use serde::Serialize;
+
+use codex_protocol::models::ResponseItem;
+
+use crate::fragment::AGENTS_MD_FRAGMENT;
+use crate::fragment::AGENTS_MD_START_MARKER;
+use crate::fragment::SKILL_FRAGMENT;
+
+pub const USER_INSTRUCTIONS_PREFIX: &str = AGENTS_MD_START_MARKER;
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[serde(rename = "user_instructions", rename_all = "snake_case")]
+pub struct UserInstructions {
+    pub directory: String,
+    pub text: String,
+}
+
+impl UserInstructions {
+    pub fn serialize_to_text(&self) -> String {
+        format!(
+            "{prefix}{directory}\n\n<INSTRUCTIONS>\n{contents}\n{suffix}",
+            prefix = AGENTS_MD_FRAGMENT.start_marker(),
+            directory = self.directory,
+            contents = self.text,
+            suffix = AGENTS_MD_FRAGMENT.end_marker(),
+        )
+    }
+}
+
+impl From<UserInstructions> for ResponseItem {
+    fn from(ui: UserInstructions) -> Self {
+        AGENTS_MD_FRAGMENT.into_message(ui.serialize_to_text())
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[serde(rename = "skill_instructions", rename_all = "snake_case")]
+pub struct SkillInstructions {
+    pub name: String,
+    pub path: String,
+    pub contents: String,
+}
+
+impl From<SkillInstructions> for ResponseItem {
+    fn from(si: SkillInstructions) -> Self {
+        SKILL_FRAGMENT.into_message(SKILL_FRAGMENT.wrap(format!(
+            "<name>{}</name>\n<path>{}</path>\n{}",
+            si.name, si.path, si.contents
+        )))
+    }
+}
+
+#[cfg(test)]
+#[path = "user_instructions_tests.rs"]
+mod tests;