feat(worker): add wait_for_task_change MCP tool

Replaces the list handler's Start-Sleep + blind get_task poll (Phase 3
of the merge-helper prompt) with a blocking MCP tool that returns as
soon as a task leaves Queued/Running, or times out. Implemented as an
async DB poll (short-lived DbContext, 500ms delay, no held connection)
rather than hooking HubBroadcaster, keeping the existing broadcast
callers untouched. timeoutSeconds is clamped server-side to 170s, under
the list handler's 200s MCP_TOOL_TIMEOUT.

src/ClaudeDo.Worker/External/TaskWaitMcpTools.cs

@@ -0,0 +1,83 @@
+using System.ComponentModel;
+using ClaudeDo.Data;
+using Microsoft.EntityFrameworkCore;
+using ModelContextProtocol.Server;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Worker.External;
+
+public sealed record TaskStatusChangeDto(string TaskId, string Status);
+public sealed record WaitForTaskChangeResult(IReadOnlyList<TaskStatusChangeDto> Changed, bool TimedOut);
+
+[McpServerToolType]
+public sealed class TaskWaitMcpTools
+{
+    // InteractiveLaunchSpecService sets MCP_TOOL_TIMEOUT=200000ms for the list handler
+    // session; this cap leaves a ~30s margin so the tool itself reports TimedOut instead
+    // of racing the client's own abort.
+    internal const int MaxTimeoutSeconds = 170;
+    private static readonly TimeSpan PollInterval = TimeSpan.FromMilliseconds(500);
+
+    private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
+
+    public TaskWaitMcpTools(IDbContextFactory<ClaudeDoDbContext> dbFactory)
+    {
+        _dbFactory = dbFactory;
+    }
+
+    [McpServerTool, Description(
+        "Blocks until at least one of the given tasks leaves Queued/Running, or until timeoutSeconds elapses " +
+        "(clamped server-side to 170s). Returns immediately if any task is already outside Queued/Running " +
+        "when called (an unknown id is reported as status \"NotFound\" and counts as changed). Use this instead " +
+        "of polling get_task in a loop. Result: { changed: [{ taskId, status }], timedOut }.")]
+    public async Task<WaitForTaskChangeResult> WaitForTaskChange(
+        string[] taskIds, int timeoutSeconds = 60, CancellationToken cancellationToken = default)
+    {
+        if (taskIds.Length == 0)
+            throw new ArgumentException("taskIds must not be empty.", nameof(taskIds));
+
+        var timeout = TimeSpan.FromSeconds(Math.Clamp(timeoutSeconds, 1, MaxTimeoutSeconds));
+        using var timeoutCts = new CancellationTokenSource(timeout);
+        using var linked = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, timeoutCts.Token);
+
+        try
+        {
+            while (true)
+            {
+                var changed = await CheckOnceAsync(taskIds, linked.Token);
+                if (changed.Count > 0)
+                    return new WaitForTaskChangeResult(changed, TimedOut: false);
+
+                await Task.Delay(PollInterval, linked.Token);
+            }
+        }
+        catch (OperationCanceledException) when (!cancellationToken.IsCancellationRequested)
+        {
+            return new WaitForTaskChangeResult(Array.Empty<TaskStatusChangeDto>(), TimedOut: true);
+        }
+    }
+
+    private async Task<IReadOnlyList<TaskStatusChangeDto>> CheckOnceAsync(string[] taskIds, CancellationToken ct)
+    {
+        await using var ctx = await _dbFactory.CreateDbContextAsync(ct);
+        var rows = await ctx.Tasks
+            .AsNoTracking()
+            .Where(t => taskIds.Contains(t.Id))
+            .Select(t => new { t.Id, t.Status })
+            .ToListAsync(ct);
+
+        var byId = rows.ToDictionary(r => r.Id, r => r.Status);
+        var result = new List<TaskStatusChangeDto>();
+        foreach (var id in taskIds)
+        {
+            if (!byId.TryGetValue(id, out var status))
+            {
+                result.Add(new TaskStatusChangeDto(id, "NotFound"));
+                continue;
+            }
+            if (status != TaskStatus.Queued && status != TaskStatus.Running)
+                result.Add(new TaskStatusChangeDto(id, status.ToString()));
+        }
+        return result;
+    }
+}