feat(worker): Online Inbox sync engine (Phase 1)

Optional, opt-in (online_inbox.enabled, default false → zero network).
Worker-side reconcile loop: pull web-created tasks down as Idle, push the
list catalog and the Idle backlog mirror up. Auth behind IOnlineAuthProvider
(StaticTokenAuthProvider default; ZitadelAuthProvider stubbed for Phase 2).
DPAPI refresh-token store. 35 tests, no real network/Zitadel/Claude.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Worker/Online/ZitadelAuthProvider.cs

@@ -0,0 +1,13 @@
+using ClaudeDo.Worker.Online.Interfaces;
+
+namespace ClaudeDo.Worker.Online;
+
+// TODO(online-inbox): wire the Zitadel package once client config is known (Phase 2).
+// Replace this stub with a real implementation that uses OnlineTokenStore to read the
+// refresh token and exchanges it for an access token via the Zitadel OIDC endpoint,
+// caching the access token until near expiry.
+public sealed class ZitadelAuthProvider : IOnlineAuthProvider
+{
+    public Task<string?> GetAccessTokenAsync(CancellationToken ct = default)
+        => Task.FromResult<string?>(null);
+}