Refactor AGENTS.md discovery into AgentsMdManager (#18035)

Encapsulate Agents MD processing a bit and drop user_instructions_path
from config.

codex-rs/app-server/src/codex_message_processor.rs

@@ -617,17 +617,9 @@ pub(crate) struct CodexMessageProcessorArgs {
 
 impl CodexMessageProcessor {
     async fn instruction_sources_from_config(config: &Config) -> Vec<AbsolutePathBuf> {
-        let mut paths: Vec<AbsolutePathBuf> =
-            config.user_instructions_path.iter().cloned().collect();
-        match codex_core::discover_project_doc_paths(config, LOCAL_FS.as_ref()).await {
-            Ok(project_doc_paths) => {
-                paths.extend(project_doc_paths);
-            }
-            Err(err) => {
-                tracing::warn!(error = %err, "failed to discover project docs for thread response");
-            }
-        }
-        paths
+        codex_core::AgentsMdManager::new(config)
+            .instruction_sources(LOCAL_FS.as_ref())
+            .await
     }
 
     pub(crate) fn handle_config_mutation(&self) {