Mark goals feature as experimental (#20083)

## Why

The `goals` feature flag is ready to move out of the hidden
under-development bucket and into the user-facing experimental surface.
Marking it experimental lets users discover it through the experimental
features UI while still making clear that it is opt-in.

## What changed

- Changed `goals` from `Stage::UnderDevelopment` to
`Stage::Experimental` in `codex-rs/features/src/lib.rs`.
- Added experimental menu metadata for the feature with the description
`Set a persistent goal Codex can continue over time`.

## Verification

- `cargo test -p codex-features`

codex-rs/features/src/lib.rs

@@ -975,7 +975,11 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::Goals,
         key: "goals",
-        stage: Stage::UnderDevelopment,
+        stage: Stage::Experimental {
+            name: "Goals",
+            menu_description: "Set a persistent goal Codex can continue over time",
+            announcement: "",
+        },
         default_enabled: false,
     },
     FeatureSpec {