feat(list-handler): hand off to a fresh session after Phase 2

The merge-helper ("Let Claude handle it") system prompt now calls a new
handoff_list_handler MCP tool once every surviving task is enhanced,
instead of continuing into Phases 3-5 in the same session -- avoiding
paying for Phases 0-2's dedupe/rewrite context on every polling round of
the run/review/merge phases.

The tool broadcasts HandoffRequested; Mission Control opens a second
ConPTY tile for the SAME handler task id (no new task, HandlerBaseCommit
untouched) running a fresh handoff brief that starts at Phase 3. The
original tile stays open. Adds PromptKind.MergeHelperHandoff,
InteractiveLaunchSpecService.BuildForMergeHelperHandoffAsync, and the
GetMergeHelperHandoffLaunchSpec hub method.

tests/ClaudeDo.Worker.Tests/Runner/InteractiveLaunchSpecServiceTests.cs

@@ -7,6 +7,7 @@ using ClaudeDo.Worker.Planning;
 using ClaudeDo.Worker.Runner;
 using ClaudeDo.Worker.Skills;
 using ClaudeDo.Worker.Tests.Infrastructure;
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging.Abstractions;
 using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
@@ -697,6 +698,108 @@ public sealed class InteractiveLaunchSpecServiceTests : IDisposable
         Assert.Contains(t1, created.Description);
     }
 
+    // ── BuildForMergeHelperHandoffAsync ──
+
+    [Fact]
+    public async Task BuildForMergeHelperHandoffAsync_EmptySurvivingTaskIds_ThrowsInvalidOperation()
+    {
+        var listId = await SeedListAsync(workingDir: _tempDir);
+        var handlerTaskId = Guid.NewGuid().ToString();
+        await SeedTaskAsync(handlerTaskId, listId, TaskStatus.Idle);
+
+        var svc = BuildService();
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => svc.BuildForMergeHelperHandoffAsync(handlerTaskId, Array.Empty<string>(), CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task BuildForMergeHelperHandoffAsync_UnknownHandlerTask_ThrowsKeyNotFound()
+    {
+        await Assert.ThrowsAsync<KeyNotFoundException>(
+            () => BuildService().BuildForMergeHelperHandoffAsync("no-such-task", new[] { "x" }, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task BuildForMergeHelperHandoffAsync_UnknownSurvivingTask_ThrowsKeyNotFound()
+    {
+        var listId = await SeedListAsync(workingDir: _tempDir);
+        var handlerTaskId = Guid.NewGuid().ToString();
+        await SeedTaskAsync(handlerTaskId, listId, TaskStatus.Idle);
+
+        var svc = BuildService();
+        await Assert.ThrowsAsync<KeyNotFoundException>(
+            () => svc.BuildForMergeHelperHandoffAsync(handlerTaskId, new[] { "no-such-task" }, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task BuildForMergeHelperHandoffAsync_ListWithoutExistingWorkingDir_ThrowsInvalidOperation()
+    {
+        var listId = await SeedListAsync(workingDir: Path.Combine(_tempDir, "gone"));
+        var handlerTaskId = Guid.NewGuid().ToString();
+        await SeedTaskAsync(handlerTaskId, listId, TaskStatus.Idle);
+        var survivor = Guid.NewGuid().ToString();
+        await SeedTaskAsync(survivor, listId, TaskStatus.WaitingForReview);
+
+        var svc = BuildService();
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => svc.BuildForMergeHelperHandoffAsync(handlerTaskId, new[] { survivor }, CancellationToken.None));
+        Assert.Contains("working directory", ex.Message);
+    }
+
+    [Fact]
+    public async Task BuildForMergeHelperHandoffAsync_ReusesHandlerTaskId_NoNewTaskCreated()
+    {
+        var repo = Path.Combine(_tempDir, "repoHandoff");
+        Directory.CreateDirectory(repo);
+
+        var listId = await SeedListAsync(workingDir: repo, name: "Alpha");
+        var handlerTaskId = Guid.NewGuid().ToString();
+        await SeedTaskAsync(handlerTaskId, listId, TaskStatus.Idle, title: "List handler: Alpha");
+        var survivor = Guid.NewGuid().ToString();
+        await SeedTaskAsync(survivor, listId, TaskStatus.WaitingForReview, title: "Survivor task");
+
+        var beforeCount = await CountTasksAsync();
+
+        var svc = BuildService();
+        var spec = await svc.BuildForMergeHelperHandoffAsync(handlerTaskId, new[] { survivor }, CancellationToken.None);
+        var sessionDir = TrackSessionDir(spec);
+
+        var afterCount = await CountTasksAsync();
+        Assert.Equal(beforeCount, afterCount);
+
+        Assert.Equal(repo, spec.Cwd);
+        Assert.Equal(_claudeStubPath, spec.Exe);
+
+        var args = spec.Args.ToList();
+        var atIdx = args.IndexOf("--allowedTools");
+        Assert.Equal("mcp__claudedo__*,Read,Grep,Glob,Edit,Bash,WebFetch,WebSearch,Skill", args[atIdx + 1]);
+
+        var appendIdx = args.IndexOf("--append-system-prompt-file");
+        var systemPromptPath = args[appendIdx + 1];
+        Assert.Equal(Path.Combine(sessionDir, "system-prompt.md"), systemPromptPath);
+        Assert.Equal(PromptFiles.ReadOrDefault(PromptKind.MergeHelper), File.ReadAllText(systemPromptPath));
+
+        var kickoff = args[^1];
+        var handoffPath = Path.Combine(sessionDir, "handoff.md");
+        Assert.Contains(handoffPath, kickoff);
+        Assert.DoesNotContain('\n', kickoff);
+
+        var handoffBrief = File.ReadAllText(handoffPath);
+        Assert.Contains("Scope: List: Alpha", handoffBrief);
+        Assert.Contains($"Repo: {repo}", handoffBrief);
+        Assert.Contains("Survivor task", handoffBrief);
+        Assert.Contains(survivor, handoffBrief);
+        Assert.Contains("phase 3", handoffBrief, StringComparison.OrdinalIgnoreCase);
+
+        Assert.Equal("200000", spec.Env["MCP_TOOL_TIMEOUT"]);
+    }
+
+    private async Task<int> CountTasksAsync()
+    {
+        using var ctx = _db.CreateContext();
+        return await ctx.Tasks.CountAsync();
+    }
+
     [Fact]
     public void BuildPlanningStart_MapsPlanningArgsAndEnv()
     {