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>

tests/ClaudeDo.Worker.Tests/Online/StaticTokenAuthProviderTests.cs

@@ -0,0 +1,30 @@
+using ClaudeDo.Worker.Online;
+
+namespace ClaudeDo.Worker.Tests.Online;
+
+public sealed class StaticTokenAuthProviderTests
+{
+    [Fact]
+    public async Task WithToken_Returns_Token()
+    {
+        var provider = new StaticTokenAuthProvider("my-token");
+        var result = await provider.GetAccessTokenAsync();
+        Assert.Equal("my-token", result);
+    }
+
+    [Fact]
+    public async Task WithNull_Returns_Null()
+    {
+        var provider = new StaticTokenAuthProvider(null);
+        var result = await provider.GetAccessTokenAsync();
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public async Task Default_Returns_Null()
+    {
+        var provider = new StaticTokenAuthProvider();
+        var result = await provider.GetAccessTokenAsync();
+        Assert.Null(result);
+    }
+}