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/Config/WorkerConfig.cs

@@ -44,6 +44,10 @@ public sealed class WorkerConfig
     [JsonPropertyName("online_inbox")]
     public OnlineInboxConfig OnlineInbox { get; set; } = new();
 
+    /// <summary>Poll interval for the OAuth usage monitor. Clamped to a minimum of 15s on load.</summary>
+    [JsonPropertyName("usage_poll_interval_seconds")]
+    public int UsagePollIntervalSeconds { get; set; } = 60;
+
     public static string DefaultConfigPath =>
         Path.Combine(Paths.AppDataRoot(), "worker.config.json");
 
@@ -71,6 +75,7 @@ public sealed class WorkerConfig
         cfg.SandboxRoot = Paths.Expand(cfg.SandboxRoot);
         cfg.LogRoot = Paths.Expand(cfg.LogRoot);
         cfg.CentralWorktreeRoot = Paths.Expand(cfg.CentralWorktreeRoot);
+        cfg.UsagePollIntervalSeconds = Math.Max(15, cfg.UsagePollIntervalSeconds);
 
         return cfg;
     }