feat(watcher): add idle detection with auto-pause/resume

When user is idle beyond IdleTimeoutMs, automatically pauses the active
task via the API. When user returns, resumes the task if it's still paused.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

WindowWatcher/Worker.cs

@@ -15,12 +15,15 @@ public class Worker(
     private string _lastAppName = string.Empty;
     private string _lastWindowTitle = string.Empty;
     private DateTime _lastChangeTime = DateTime.MinValue;
+    private bool _isIdle;
+    private int? _pausedTaskId;
 
     protected override async Task ExecuteAsync(CancellationToken stoppingToken)
     {
         var config = options.Value;
-        logger.LogInformation("WindowWatcher started. Polling every {Interval}ms, debounce {Debounce}ms",
-            config.PollIntervalMs, config.DebounceMs);
+        logger.LogInformation(
+            "WindowWatcher started. Polling every {Interval}ms, debounce {Debounce}ms, idle timeout {IdleTimeout}ms",
+            config.PollIntervalMs, config.DebounceMs, config.IdleTimeoutMs);
 
         while (!stoppingToken.IsCancellationRequested)
         {
@@ -67,6 +70,21 @@ public class Worker(
                     _lastWindowTitle = windowTitle;
                     _lastChangeTime = now;
                 }
+
+                // Idle detection
+                var idleTime = NativeMethods.GetIdleTime();
+                if (!_isIdle && idleTime.TotalMilliseconds >= config.IdleTimeoutMs)
+                {
+                    _isIdle = true;
+                    logger.LogInformation("User idle for {IdleTime}, pausing active task", idleTime);
+                    await PauseActiveTaskAsync(ct: stoppingToken);
+                }
+                else if (_isIdle && idleTime.TotalMilliseconds < config.IdleTimeoutMs)
+                {
+                    _isIdle = false;
+                    logger.LogInformation("User returned from idle");
+                    await ResumeIdlePausedTaskAsync(ct: stoppingToken);
+                }
             }
             catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
             {
@@ -108,4 +126,80 @@ public class Worker(
             logger.LogWarning(ex, "Failed to report context event to API");
         }
     }
+
+    private async Task PauseActiveTaskAsync(CancellationToken ct)
+    {
+        try
+        {
+            var client = httpClientFactory.CreateClient("TaskTrackerApi");
+
+            // Get the active task
+            var response = await client.GetFromJsonAsync<ApiResponse<ActiveTaskDto>>(
+                "/api/tasks/active", ct);
+
+            if (response?.Data is null)
+            {
+                logger.LogDebug("No active task to pause");
+                return;
+            }
+
+            _pausedTaskId = response.Data.Id;
+
+            // Pause it
+            var pauseResponse = await client.PutAsJsonAsync(
+                $"/api/tasks/{_pausedTaskId}/pause",
+                new { note = "Auto-paused: idle timeout" }, ct);
+
+            if (pauseResponse.IsSuccessStatusCode)
+                logger.LogInformation("Auto-paused task {TaskId}", _pausedTaskId);
+            else
+                logger.LogWarning("Failed to pause task {TaskId}: {Status}", _pausedTaskId, pauseResponse.StatusCode);
+        }
+        catch (Exception ex)
+        {
+            logger.LogWarning(ex, "Failed to pause active task on idle");
+        }
+    }
+
+    private async Task ResumeIdlePausedTaskAsync(CancellationToken ct)
+    {
+        if (_pausedTaskId is null)
+            return;
+
+        var taskId = _pausedTaskId.Value;
+        _pausedTaskId = null;
+
+        try
+        {
+            var client = httpClientFactory.CreateClient("TaskTrackerApi");
+
+            // Check the task is still paused (user may have manually switched tasks)
+            var response = await client.GetFromJsonAsync<ApiResponse<ActiveTaskDto>>(
+                $"/api/tasks/{taskId}", ct);
+
+            if (response?.Data is null || response.Data.Status != "Paused")
+            {
+                logger.LogDebug("Task {TaskId} is no longer paused, skipping auto-resume", taskId);
+                return;
+            }
+
+            // Resume it
+            var resumeResponse = await client.PutAsJsonAsync(
+                $"/api/tasks/{taskId}/resume",
+                new { note = "Auto-resumed: user returned" }, ct);
+
+            if (resumeResponse.IsSuccessStatusCode)
+                logger.LogInformation("Auto-resumed task {TaskId}", taskId);
+            else
+                logger.LogWarning("Failed to resume task {TaskId}: {Status}", taskId, resumeResponse.StatusCode);
+        }
+        catch (Exception ex)
+        {
+            logger.LogWarning(ex, "Failed to resume task {TaskId} after idle", taskId);
+        }
+    }
 }
+
+internal record ActiveTaskDto(int Id, string Status);
+
+internal record ApiResponse<T>(bool Success, T? Data, string? Error);