Organize context fragments (#18794)

Organize context fragments under `core/context`. Implement same trait on
all of them.

codex-rs/core/src/context/user_instructions.rs

@@ -0,0 +1,17 @@
+use super::ContextualUserFragment;
+
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct UserInstructions {
+    pub(crate) directory: String,
+    pub(crate) text: String,
+}
+
+impl ContextualUserFragment for UserInstructions {
+    const ROLE: &'static str = "user";
+    const START_MARKER: &'static str = "# AGENTS.md instructions for ";
+    const END_MARKER: &'static str = "</INSTRUCTIONS>";
+
+    fn body(&self) -> String {
+        format!("{}\n\n<INSTRUCTIONS>\n{}", self.directory, self.text)
+    }
+}