feat(worker): add OAuth usage client + poller

Adds Usage/ with ClaudeOAuthUsageClient (reads the access token Claude
Code keeps fresh at ~/.claude/.credentials.json, calls the Anthropic
OAuth usage endpoint, defensively parses buckets/limits), UsageState
(threadsafe last-good-snapshot holder that never regresses on
failure), and UsageMonitorService (BackgroundService polling on the
new usage_poll_interval_seconds config, min 15s, one poll at startup,
warns at most once per distinct error).

tests/ClaudeDo.Worker.Tests/Usage/UsageStateTests.cs

@@ -0,0 +1,48 @@
+using ClaudeDo.Worker.Usage;
+
+namespace ClaudeDo.Worker.Tests.Usage;
+
+public sealed class UsageStateTests
+{
+    private static UsageSnapshot MakeSnapshot(double fiveHourUtilization) => new(
+        new UsageBucket(fiveHourUtilization, null),
+        null,
+        [],
+        DateTime.UtcNow);
+
+    [Fact]
+    public void ReportSuccess_SetsSnapshotAndClearsError()
+    {
+        var state = new UsageState();
+        state.ReportFailure("boom", DateTime.UtcNow);
+
+        state.ReportSuccess(MakeSnapshot(42));
+
+        Assert.NotNull(state.Snapshot);
+        Assert.Equal(42, state.Snapshot!.FiveHour!.Utilization);
+        Assert.Null(state.LastError);
+    }
+
+    [Fact]
+    public void ReportFailure_AfterSuccess_KeepsOldSnapshotButSetsError()
+    {
+        var state = new UsageState();
+        state.ReportSuccess(MakeSnapshot(10));
+
+        state.ReportFailure("network down", DateTime.UtcNow);
+
+        Assert.NotNull(state.Snapshot);
+        Assert.Equal(10, state.Snapshot!.FiveHour!.Utilization);
+        Assert.Equal("network down", state.LastError);
+    }
+
+    [Fact]
+    public void NoAttemptYet_SnapshotAndErrorAreNull()
+    {
+        var state = new UsageState();
+
+        Assert.Null(state.Snapshot);
+        Assert.Null(state.LastError);
+        Assert.Null(state.LastAttemptUtc);
+    }
+}