FN-5827: derive per-subtask working branches in shared planning mode

Ensure planning subtask creation always assigns distinct per-task working branches while preserving shared merge-target grouping.

- route planning subtask branch assignment through resolveEntryPointBranchAssignment for both direct and merged subtask creation paths
- keep shared assignment mode grouped by branch context while deriving unique working branches from the planning branch
- update modal copy to clarify shared mode semantics (shared merge target + per-task branches)
- adjust planning/tasks route tests to assert derived per-task branch names and shared branch-context metadata

Files changed:
 .../app/components/SubtaskBreakdownModal.tsx       |  4 +-
 .../src/__tests__/routes-planning.test.ts          | 51 ++++++++++++++++++----
 .../dashboard/src/__tests__/routes-tasks.test.ts   |  2 +-
 .../src/routes/register-planning-subtask-routes.ts | 18 +++++---
 4 files changed, 57 insertions(+), 18 deletions(-)

Fusion-Task-Id: FN-5827

Fusion-Task-Lineage: 5d6d162f-b89f-49c5-b962-be41332547fe

packages/dashboard/app/components/SubtaskBreakdownModal.tsx

@@ -731,8 +731,8 @@ export function SubtaskBreakdownModal({ isOpen, onClose, initialDescription, onT
                     <div className="form-group">
                       <label>Planning branch mode</label>
                       <select value={branchAssignmentMode} onChange={(event) => setBranchAssignmentMode(event.target.value as typeof branchAssignmentMode)} disabled={view.type === "creating"}>
-                        <option value="shared">Shared branch for all subtasks</option>
-                        <option value="per-task-derived">Per-task branch derived from planning branch</option>
+                        <option value="shared">Shared merge target — subtasks run on their own branches</option>
+                        <option value="per-task-derived">Per-task branches derived from planning branch</option>
                       </select>
                     </div>
                   </div>