fix: per-task timer exclusivity, not global

Multiple tasks can each have running timers simultaneously — this supports
multi-agent workflows where different agents track different tasks.

Removed:
- getRunningTimerTask() method (global scan)
- Auto-stop of other tasks' timers in startTimer()
- otherRunningTask UI check that hid the Start button

Kept: per-task guard (can't start a timer on a task that already has one running).

server/src/routes/task-time.ts

@@ -27,15 +27,8 @@ router.get(
 router.post(
   '/:id/time/start',
   asyncHandler(async (req, res) => {
-    const { task, stoppedTaskId } = await taskService.startTimer(req.params.id as string);
-
-    // Broadcast WS event for the auto-stopped task (if any) so other clients refresh
-    if (stoppedTaskId) {
-      broadcastTaskChange('updated', stoppedTaskId);
-    }
-    // Broadcast WS event for the started task
+    const task = await taskService.startTimer(req.params.id as string);
     broadcastTaskChange('updated', task.id);
-
     res.json(task);
   })
 );

server/src/services/task-service.ts

@@ -680,48 +680,21 @@ export class TaskService {
 
   // ============ Time Tracking Methods ============
 
-  /**
-   * Find the task that currently has a running timer (if any).
-   */
-  async getRunningTimerTask(): Promise<Task | null> {
-    await this.initCache();
-    for (const task of this.cache.values()) {
-      if (task.timeTracking?.isRunning) {
-        return task;
-      }
-    }
-    return null;
-  }
-
   /**
    * Start a timer for a task.
-   * Enforces global exclusivity: only one timer may run at a time.
-   * If another task has a running timer, it is auto-stopped first.
-   * Returns { task, stoppedTaskId? } so the caller can broadcast both.
+   * Per-task exclusivity: only one timer per task (but multiple tasks can
+   * each have their own running timer — supports multi-agent workflows).
    */
-  async startTimer(taskId: string): Promise<{ task: Task; stoppedTaskId?: string }> {
+  async startTimer(taskId: string): Promise<Task> {
     const task = await this.getTask(taskId);
     if (!task) {
       throw new NotFoundError('Task not found');
     }
 
-    // Check if timer is already running on THIS task
     if (task.timeTracking?.isRunning) {
       throw new ConflictError('Timer is already running for this task');
     }
 
-    // Global exclusivity: stop any other running timer first
-    let stoppedTaskId: string | undefined;
-    const runningTask = await this.getRunningTimerTask();
-    if (runningTask && runningTask.id !== taskId) {
-      await this.stopTimer(runningTask.id);
-      stoppedTaskId = runningTask.id;
-      log.info(
-        { stoppedTaskId, startedTaskId: taskId },
-        'Auto-stopped timer for global exclusivity'
-      );
-    }
-
     const entryId = `time_${Date.now()}_${Math.random().toString(36).slice(2, 8)}`;
     const now = new Date().toISOString();
 
@@ -737,8 +710,7 @@ export class TaskService {
       activeEntryId: entryId,
     };
 
-    const updated = (await this.updateTask(taskId, { timeTracking })) as Task;
-    return { task: updated, stoppedTaskId };
+    return (await this.updateTask(taskId, { timeTracking })) as Task;
   }
 
   /**

web/src/components/task/TimeTrackingSection.tsx

@@ -103,9 +103,6 @@ export function TimeTrackingSection({ task }: TimeTrackingSectionProps) {
   const entries = timeTracking?.entries ?? [];
   const activeEntry = entries.find((e) => e.id === timeTracking?.activeEntryId);
 
-  // Global exclusivity: is another task's timer running?
-  const otherRunningTask = allTasks?.find((t) => t.id !== task.id && t.timeTracking?.isRunning);
-
   // ── Handlers ──
 
   const handleStartStop = async () => {
@@ -218,13 +215,8 @@ export function TimeTrackingSection({ task }: TimeTrackingSectionProps) {
                   </>
                 )}
               </Button>
-            ) : otherRunningTask ? (
-              /* Another task has the active timer — Start is blocked */
-              <span className="text-xs text-muted-foreground italic">
-                Timer active on another task
-              </span>
             ) : (
-              /* No timer running anywhere — show Start */
+              /* Timer not running on this task — show Start */
               <Button variant="default" size="sm" onClick={handleStartStop} disabled={busy}>
                 {busy ? (
                   <Loader2 className="h-4 w-4 animate-spin" />