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.

src/ClaudeDo.Data/PromptFiles.cs

@@ -4,7 +4,7 @@ using System.Text.Json;
 
 namespace ClaudeDo.Data;
 
-public enum PromptKind { System, Planning, PlanningInitial, Retry, DailyPrep, WeeklyReport, ImprovementChild, Refine, MergeHelper, MergeHelperInitial }
+public enum PromptKind { System, Planning, PlanningInitial, Retry, DailyPrep, WeeklyReport, ImprovementChild, Refine, MergeHelper, MergeHelperInitial, MergeHelperHandoff }
 
 /// <summary>
 /// How a prompt kind's on-disk override (if any) relates to the bundled default.
@@ -40,6 +40,7 @@ public static class PromptFiles
         PromptKind.Refine => "refine.md",
         PromptKind.MergeHelper => "merge-helper-system.md",
         PromptKind.MergeHelperInitial => "merge-helper-initial.md",
+        PromptKind.MergeHelperHandoff => "merge-helper-handoff.md",
         _ => throw new ArgumentOutOfRangeException(nameof(kind))
     };
 
@@ -212,6 +213,7 @@ public static class PromptFiles
         PromptKind.Refine => RefineDefault,
         PromptKind.MergeHelper => MergeHelperDefault,
         PromptKind.MergeHelperInitial => MergeHelperInitialDefault,
+        PromptKind.MergeHelperHandoff => MergeHelperHandoffDefault,
         _ => ""
     };
 
@@ -425,6 +427,9 @@ public static class PromptFiles
 
         If a task visibly bundles several independent features, or has a blocker that is not resolved by anything in its own description, do not force it into one description. Propose splitting it to the user; if they agree, create the pieces with add_task/add_subtask and only move the pieces the user confirmed into "surviving tasks" for the phases below. Split only what the task already asks for — the "do not invent requirements" rule still applies.
 
+        ## Handoff
+        Once every surviving task is enhanced, call handoff_list_handler with this session's task id and the surviving task ids, in the order you intend to run them. That opens a fresh session to carry out phases 3–5 with just that list, without dragging along this session's dedupe/rewrite context. Say a short goodbye line, then stop — do not continue into phase 3 yourself.
+
         ## Phase 3 — Run
         Do NOT use run_task_now for a batch — there is a single override slot and the second call fails with "override slot busy".
 
@@ -485,6 +490,21 @@ public static class PromptFiles
         When every task is handled, print the summary.
         """;
 
+    private const string MergeHelperHandoffDefault = """
+        # List handler handoff
+
+        Scope: {scope}
+        Repo: {repo}
+
+        A prior session already read, deduped and enhanced this list's tasks. Pick up at phase 3
+        for the tasks below — their descriptions are already sharpened, so don't redo phases 0–2.
+
+        {tasks}
+
+        Start with phase 3 (run), continuing through review/merge and the summary as your
+        instructions describe.
+        """;
+
     private const string WeeklyReportDefault = """
         You are generating a concise weekly standup report for a software developer,
         covering {start} to {end}.