[codex-analytics] enable general analytics by default (#17389)

## Summary
- Make GeneralAnalytics stable and enabled by default.
- Update feature tests and app-server lifecycle fixtures for explicit
general_analytics=false.
- Keep app-server integration tests isolated from host managed config so
explicit feature fixtures are deterministic.

## Validation
- cargo test -p codex-features
- cargo test -p codex-app-server general_analytics (matched 0 tests)
- cargo test -p codex-app-server thread_start_
- cargo test -p codex-app-server thread_fork_
- cargo test -p codex-app-server thread_resume_
- cargo test -p codex-app-server
config_read_includes_system_layer_and_overrides

codex-rs/features/src/lib.rs

@@ -660,8 +660,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::GeneralAnalytics,
         key: "general_analytics",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::Sqlite,