FN-5829: add shared feature branch option for new tasks

Allow new tasks to target a shared branch-group feature branch during creation.

- Add branch-group aware branch selection handling in task workflow routes.
- Update store/types/db wiring to support and persist shared feature-branch targeting.
- Extend dashboard task creation UI/tests and route tests to cover the new option.
- Add a changeset for @runfusion/fusion documenting the new CLI/task behavior.

Files changed:
 .changeset/fn-5829-shared-feature-branch-option.md |   7 ++
 .../src/commands/__tests__/task-lifecycle.test.ts  |   2 +-
 .../core/src/__tests__/branch-group-store.test.ts  |  26 ++++++
 packages/core/src/db.ts                            |   4 +-
 packages/core/src/store.ts                         |   9 +-
 packages/core/src/types.ts                         |   2 +-
 packages/dashboard/app/components/NewTaskModal.tsx |   2 +-
 packages/dashboard/app/components/TaskForm.tsx     |   9 +-
 .../app/components/__tests__/NewTaskModal.test.tsx |  37 ++++++++
 .../src/__tests__/branch-selection.test.ts         |  12 +++
 .../dashboard/src/__tests__/mission-e2e.test.ts    |   8 +-
 .../src/__tests__/routes-planning.test.ts          |   6 +-
 .../src/__tests__/routes-tasks.test.ts             | 102 +++++++++++++++++++++
 packages/dashboard/src/routes/branch-selection.ts  |  19 +++-
 .../src/routes/register-task-workflow-routes.ts    |  23 +++--
 15 files changed, 242 insertions(+), 26 deletions(-)

Fusion-Task-Id: FN-5829

Fusion-Task-Lineage: 7d69acc8-19a3-4112-9bf2-5ac7b5e5a929

packages/cli/src/commands/__tests__/task-lifecycle.test.ts

@@ -37,7 +37,7 @@ interface MockTask {
   baseBranch?: string;
   branchContext?: {
     groupId: string;
-    source: "planning" | "mission";
+    source: "planning" | "mission" | "new-task";
     assignmentMode: "shared" | "per-task-derived";
     inheritedBaseBranch?: string;
   };