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/fragment.rs

@@ -0,0 +1,61 @@
+use codex_protocol::models::ContentItem;
+use codex_protocol::models::ResponseItem;
+
+pub(crate) const AGENTS_MD_START_MARKER: &str = "# AGENTS.md instructions for ";
+pub(crate) const AGENTS_MD_END_MARKER: &str = "</INSTRUCTIONS>";
+pub(crate) const SKILL_OPEN_TAG: &str = "<skill>";
+pub(crate) const SKILL_CLOSE_TAG: &str = "</skill>";
+
+#[derive(Clone, Copy)]
+pub struct ContextualUserFragmentDefinition {
+    start_marker: &'static str,
+    end_marker: &'static str,
+}
+
+impl ContextualUserFragmentDefinition {
+    pub const fn new(start_marker: &'static str, end_marker: &'static str) -> Self {
+        Self {
+            start_marker,
+            end_marker,
+        }
+    }
+
+    pub fn matches_text(&self, text: &str) -> bool {
+        let trimmed = text.trim_start();
+        let starts_with_marker = trimmed
+            .get(..self.start_marker.len())
+            .is_some_and(|candidate| candidate.eq_ignore_ascii_case(self.start_marker));
+        let trimmed = trimmed.trim_end();
+        let ends_with_marker = trimmed
+            .get(trimmed.len().saturating_sub(self.end_marker.len())..)
+            .is_some_and(|candidate| candidate.eq_ignore_ascii_case(self.end_marker));
+        starts_with_marker && ends_with_marker
+    }
+
+    pub const fn start_marker(&self) -> &'static str {
+        self.start_marker
+    }
+
+    pub const fn end_marker(&self) -> &'static str {
+        self.end_marker
+    }
+
+    pub fn wrap(&self, body: String) -> String {
+        format!("{}\n{}\n{}", self.start_marker, body, self.end_marker)
+    }
+
+    pub fn into_message(self, text: String) -> ResponseItem {
+        ResponseItem::Message {
+            id: None,
+            role: "user".to_string(),
+            content: vec![ContentItem::InputText { text }],
+            end_turn: None,
+            phase: None,
+        }
+    }
+}
+
+pub const AGENTS_MD_FRAGMENT: ContextualUserFragmentDefinition =
+    ContextualUserFragmentDefinition::new(AGENTS_MD_START_MARKER, AGENTS_MD_END_MARKER);
+pub const SKILL_FRAGMENT: ContextualUserFragmentDefinition =
+    ContextualUserFragmentDefinition::new(SKILL_OPEN_TAG, SKILL_CLOSE_TAG);