FN-5822: wire shared branch-group visibility and merge controls

Add dashboard and API support for viewing shared branch groups and triggering group merge actions.

- add branch-group API routes and register them in integrated routers
- extend dashboard legacy API client with branch-group fetch and merge-control actions
- add BranchGroupCard UI + styles and surface it in task/detail/subtask views
- add dashboard/API test coverage for branch-group routes and UI integration points
- document branch-group behavior and add a changeset for @runfusion/fusion

Files changed:
 .changeset/fn-5822-branch-group-dashboard.md       |   5 +
 docs/dashboard-guide.md                            |  31 ++++++
 packages/dashboard/app/api/legacy.ts               |  54 +++++++++
 .../dashboard/app/components/BranchGroupCard.css   |  83 ++++++++++++++
 .../dashboard/app/components/BranchGroupCard.tsx   | 123 +++++++++++++++++++++
 .../app/components/SubtaskBreakdownModal.tsx       |   3 +
 packages/dashboard/app/components/TaskCard.tsx     |  19 ++++
 .../dashboard/app/components/TaskDetailModal.tsx   |   4 +
 .../components/__tests__/BranchGroupCard.test.tsx  |  96 ++++++++++++++++
 .../__tests__/SubtaskBreakdownModal.test.tsx       |   1 +
 .../app/components/__tests__/TaskCard.test.tsx     |  16 ++++
 .../components/__tests__/TaskDetailModal.test.tsx  |  22 ++++
 .../src/__tests__/routes-branch-groups.test.ts     | 119 ++++++++++++++++++++
 .../src/routes/register-branch-groups-routes.ts    | 118 ++++++++++++++++++++
 .../src/routes/register-integrated-routers.ts      |  13 +++
 packages/dashboard/vitest.config.ts                |   4 +-
 16 files changed, 709 insertions(+), 2 deletions(-)

Fusion-Task-Id: FN-5822

Fusion-Task-Lineage: 199c25b8-f6ec-43b4-8fab-509f23fb5ac7

packages/dashboard/app/components/SubtaskBreakdownModal.tsx

@@ -734,6 +734,9 @@ export function SubtaskBreakdownModal({ isOpen, onClose, initialDescription, onT
                         <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>
+                      {branchAssignmentMode === "shared" && branchName.trim() && (
+                        <p className="text-muted">Grouped on shared branch <strong>{branchName.trim()}</strong></p>
+                      )}
                     </div>
                   </div>
                   {subtasks.map((subtask, index) => {