Add ThreadManager sample crate (#20141)

Summary:
- Add codex-thread-manager-sample, a one-shot binary that starts a
ThreadManager thread, submits a prompt, and prints the final assistant
output.
- Pass ThreadStore into ThreadManager::new and expose
thread_store_from_config for existing callsites.
- Build the sample Config directly with only --model and prompt inputs.

Verification:
- just fmt
- cargo check -p codex-thread-manager-sample -p codex-app-server -p
codex-mcp-server
- git diff --check

Tests: Not run per request.

codex-rs/core/src/config/managed_features.rs

@@ -27,6 +27,18 @@ pub struct ManagedFeatures {
     pinned_features: BTreeMap<Feature, bool>,
 }
 
+impl Default for ManagedFeatures {
+    fn default() -> Self {
+        Self {
+            value: ConstrainedWithSource::new(
+                Constrained::allow_any(Features::default()),
+                /*source*/ None,
+            ),
+            pinned_features: BTreeMap::new(),
+        }
+    }
+}
+
 impl ManagedFeatures {
     pub(crate) fn from_configured(
         configured_features: Features,