Add support for custom base instructions (#1645)

Allows providing custom instructions file as a config parameter and
custom instruction text via MCP tool call.

codex-rs/core/tests/test_support.rs

@@ -76,3 +76,24 @@ pub fn load_sse_fixture_with_id(path: impl AsRef<std::path::Path>, id: &str) ->
         })
         .collect()
 }
+
+#[allow(dead_code)]
+pub async fn wait_for_event<F>(
+    codex: &codex_core::Codex,
+    mut predicate: F,
+) -> codex_core::protocol::EventMsg
+where
+    F: FnMut(&codex_core::protocol::EventMsg) -> bool,
+{
+    use tokio::time::Duration;
+    use tokio::time::timeout;
+    loop {
+        let ev = timeout(Duration::from_secs(1), codex.next_event())
+            .await
+            .expect("timeout waiting for event")
+            .expect("stream ended unexpectedly");
+        if predicate(&ev.msg) {
+            return ev.msg;
+        }
+    }
+}