Make goals feature on by default and no longer experimental (#23732)

## Why

The `goals` feature is ready to be available without requiring users to
opt into experimental features. Keeping it behind the beta flag leaves
persisted thread goals and automatic goal continuation disabled by
default.

This PR also marks the goal-related app server APIs and events as no
longer experimental.

## What changed

- Mark `goals` as `Stage::Stable`.
- Enable `goals` by default in `codex-rs/features/src/lib.rs`.

codex-rs/features/src/lib.rs

@@ -1077,12 +1077,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::Goals,
         key: "goals",
-        stage: Stage::Experimental {
-            name: "Goals",
-            menu_description: "Set a persistent goal Codex can continue over time",
-            announcement: "",
-        },
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::CollaborationModes,