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/Hub/MergeHelperTaskHubTests.cs

@@ -26,6 +26,7 @@ public sealed class MergeHelperTaskHubTests : IDisposable
     private readonly TaskRepository _tasks;
     private readonly ListRepository _lists;
     private readonly List<GitRepoFixture> _repos = new();
+    private readonly List<string> _mergeHelperSessionDirs = new();
     private readonly RecordingClientProxy _proxy = new();
 
     private static bool GitAvailable => GitRepoFixture.IsGitAvailable();
@@ -42,6 +43,8 @@ public sealed class MergeHelperTaskHubTests : IDisposable
         foreach (var r in _repos) r.Dispose();
         _ctx.Dispose();
         _db.Dispose();
+        foreach (var d in _mergeHelperSessionDirs)
+            try { Directory.Delete(d, true); } catch { /* best effort */ }
     }
 
     private sealed class FakeSessionSkillRegistry : ISessionSkillRegistry
@@ -136,6 +139,37 @@ public sealed class MergeHelperTaskHubTests : IDisposable
             () => hub.CreateMergeHelperTask(new[] { "t1" }, "no-such-list", "title", "header"));
     }
 
+    // ── GetMergeHelperHandoffLaunchSpec ──
+
+    [Fact]
+    public async Task GetMergeHelperHandoffLaunchSpec_ReusesHandlerTaskId_NoNewTaskCreated()
+    {
+        var listId = await SeedListAsync(Path.GetTempPath(), name: "Alpha");
+        var handlerTask = await SeedTaskAsync(listId, TaskStatus.Idle, title: "List handler: Alpha");
+        var survivor = await SeedTaskAsync(listId, TaskStatus.WaitingForReview, title: "Survivor task");
+
+        var hub = CreateHub();
+        var spec = await hub.GetMergeHelperHandoffLaunchSpec(handlerTask.Id, new[] { survivor.Id });
+
+        var args = spec.Args.ToList();
+        var sessionDir = args[args.IndexOf("--add-dir") + 1];
+        _mergeHelperSessionDirs.Add(sessionDir);
+
+        Assert.Contains("--allowedTools", args);
+        Assert.Contains("mcp__claudedo__*,Read,Grep,Glob,Edit,Bash,WebFetch,WebSearch,Skill", args);
+
+        var kickoff = args[^1];
+        Assert.Contains(Path.Combine(sessionDir, "handoff.md"), kickoff);
+    }
+
+    [Fact]
+    public async Task GetMergeHelperHandoffLaunchSpec_UnknownHandlerTask_Throws()
+    {
+        var hub = CreateHub();
+        await Assert.ThrowsAsync<HubException>(
+            () => hub.GetMergeHelperHandoffLaunchSpec("no-such-task", new[] { "x" }));
+    }
+
     // ── SubmitTaskForReview (worktree-less branch) ──
 
     [Fact]