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.Localization/locales/en.json

@@ -291,6 +291,7 @@
     "conptyLaunchFailed": "Couldn't open interactive session: {0}",
     "conptyStarting": "Starting session…",
     "mergeHelperTitle": "Merge Helper",
+    "mergeHelperHandoffTitleSuffix": " (Handoff)",
     "mergeHelperTaskTitle": "List handler: {0}",
     "mergeHelperTaskDescriptionHeader": "Tasks handled by this run:",
     "submitForReviewFailed": "Couldn't submit for review: {0}",