fix(usage): stop 429s with an activity-dependent poll cadence + manual refresh

The usage monitor polled the undocumented OAuth usage endpoint every 60s and
earned 429s. It now polls every 5 min while any task is Running and every
15 min while idle (usage_poll_interval_active_seconds / _idle_seconds, both
clamped to >= 60; the old single usage_poll_interval_seconds key is gone).

A 429 comes back as UsageRateLimitedException carrying Retry-After and adds
exponential backoff on top, capped at 30 min and never shorter than the normal
cadence; the strike count resets on the first success. The schedule arithmetic
is the pure static UsagePollSchedule.NextDelay.

Since the idle cadence is slow on purpose, WorkerHub.RefreshUsage drives
UsageMonitorService.RefreshNowAsync behind a Refresh now button in the Usage
Monitor modal: an out-of-band poll that pushes the loop's next-due time out so
no double poll follows, with a 10s cooldown so click-spam can't earn a 429.

Staleness now measures against the slower (idle) interval so an idle worker
isn't flagged stale just for not polling.

src/ClaudeDo.Worker/Config/WorkerConfig.cs

@@ -44,9 +44,18 @@ 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;
+    /// <summary>
+    /// Usage-monitor poll interval while at least one task is Running. Clamped to a minimum
+    /// of 60s on load — the endpoint rate-limits (429) on tighter polling.
+    /// </summary>
+    [JsonPropertyName("usage_poll_interval_active_seconds")]
+    public int UsagePollIntervalActiveSeconds { get; set; } = 300;
+
+    /// <summary>
+    /// Usage-monitor poll interval while nothing is running. Clamped to a minimum of 60s on load.
+    /// </summary>
+    [JsonPropertyName("usage_poll_interval_idle_seconds")]
+    public int UsagePollIntervalIdleSeconds { get; set; } = 900;
 
     public static string DefaultConfigPath =>
         Path.Combine(Paths.AppDataRoot(), "worker.config.json");
@@ -75,7 +84,8 @@ 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);
+        cfg.UsagePollIntervalActiveSeconds = Math.Max(60, cfg.UsagePollIntervalActiveSeconds);
+        cfg.UsagePollIntervalIdleSeconds = Math.Max(60, cfg.UsagePollIntervalIdleSeconds);
 
         return cfg;
     }