feat(worker): wire list-handler phase parameter through handoff chain

Threads a nextPhase parameter (wait/merge/wait_final/merge_final, validated
by the new MergeHelperPhase) from handoff_list_handler through
HubBroadcaster/WorkerHub into InteractiveLaunchSpecService, which now picks
the next session's system prompt (MergeHelperWait/MergeHelperMerge) and
model (HandlerWaitAlias/HandlerMergeAlias) from it instead of hardcoding the
old two-phase Execute prompt -- this also fixes a build break left by the
prior prompt-split task, which removed PromptKind.MergeHelperExecute without
updating its only caller.

Also sets --model/--effort/--permission-mode explicitly for every list-handler
session (Triage included) via PermissionModeResolver instead of inheriting the
CLI's ambient model and hardcoding "auto", and adds Task to the merge-helper
allowlist so the Merge phase can delegate diff reviews to subagents.

tests/ClaudeDo.Worker.Tests/Hub/MergeHelperTaskHubTests.cs

@@ -149,14 +149,14 @@ public sealed class MergeHelperTaskHubTests : IDisposable
         var survivor = await SeedTaskAsync(listId, TaskStatus.WaitingForReview, title: "Survivor task");
 
         var hub = CreateHub();
-        var spec = await hub.GetMergeHelperHandoffLaunchSpec(handlerTask.Id, new[] { survivor.Id });
+        var spec = await hub.GetMergeHelperHandoffLaunchSpec(handlerTask.Id, new[] { survivor.Id }, "wait");
 
         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);
+        Assert.Contains("mcp__claudedo__*,Read,Grep,Glob,Edit,Bash,WebFetch,WebSearch,Skill,Task", args);
 
         var kickoff = args[^1];
         Assert.Contains(Path.Combine(sessionDir, "handoff.md"), kickoff);
@@ -167,7 +167,7 @@ public sealed class MergeHelperTaskHubTests : IDisposable
     {
         var hub = CreateHub();
         await Assert.ThrowsAsync<HubException>(
-            () => hub.GetMergeHelperHandoffLaunchSpec("no-such-task", new[] { "x" }));
+            () => hub.GetMergeHelperHandoffLaunchSpec("no-such-task", new[] { "x" }, "wait"));
     }
 
     // ── SubmitTaskForReview (worktree-less branch) ──