refactor(server): replace AppContext undefined sentinels with Feature<T>

Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019e5550-4435-7118-8393-cdcc97042178

app/server/feature.ts

@@ -0,0 +1,20 @@
+// MARK: Feature<T>
+//
+// A two-state tagged union used in place of `T | undefined` for
+// optional features on the AppContext. Carries a human-readable
+// reason when the feature is disabled so loaders can surface it (or
+// choose to ignore it).
+//
+// This is deliberately *not* a Result/Either. It does not represent
+// async readiness or retryable failure — it represents "is this
+// feature wired up at all." Services that have their own runtime
+// status (e.g. OIDC discovery) keep that on the service itself.
+
+export type Feature<T> = { state: "enabled"; value: T } | { state: "disabled"; reason: string };
+
+export const enabled = <T>(value: T): Feature<T> => ({ state: "enabled", value });
+
+export const disabled = (reason: string): Feature<never> => ({
+  state: "disabled",
+  reason,
+});