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).

src/ClaudeDo.Worker/Usage/UsageModels.cs

@@ -0,0 +1,18 @@
+namespace ClaudeDo.Worker.Usage;
+
+public sealed record UsageBucket(double Utilization, DateTimeOffset? ResetsAt);
+
+public sealed record UsageLimitRow(
+    string Kind,
+    string Group,
+    double Percent,
+    string Severity,
+    DateTimeOffset? ResetsAt,
+    string? ScopeModelDisplayName,
+    bool IsActive);
+
+public sealed record UsageSnapshot(
+    UsageBucket? FiveHour,
+    UsageBucket? SevenDay,
+    IReadOnlyList<UsageLimitRow> Limits,
+    DateTime FetchedAtUtc);