FN-5671: add branch-strategy selection to new task creation

Add branch-strategy based task branch handling from modal through task creation routes.

- add branch strategy dropdown + validation in New Task modal and TaskForm, including required branch-name handling for existing/custom-new modes
- send structured branchSelection payload from dashboard API client and cover UI/API behavior with new tests
- add server-side branch selection mode parsing, auto-new branch derivation (fusion/<task-id>-<slug>), and response updates when auto branch is generated
- update engine/worktree and dashboard route tests plus dashboard guide documentation
- add a changeset for @runfusion/fusion patch release

Files changed:
 .changeset/fn-5671-branch-strategy-dropdown.md     |  7 ++
 docs/dashboard-guide.md                            | 16 +++++
 packages/dashboard/app/__tests__/api-tasks.test.ts | 21 ++++++
 packages/dashboard/app/api/legacy.ts               |  9 +++
 packages/dashboard/app/components/NewTaskModal.css |  8 +++
 packages/dashboard/app/components/NewTaskModal.tsx | 41 +++++++++---
 packages/dashboard/app/components/TaskForm.tsx     | 30 ++++++++-
 .../app/components/__tests__/NewTaskModal.test.tsx | 77 +++++++++++++++++++---
 .../src/__tests__/branch-selection.test.ts         | 13 ++++
 .../dashboard/src/__tests__/routes-tasks.test.ts   | 72 ++++++++++++++++++++
 packages/dashboard/src/routes/branch-selection.ts  | 45 ++++++++-----
 .../src/routes/register-task-workflow-routes.ts    | 22 +++++--
 .../engine/src/__tests__/worktree-pool.test.ts     | 41 ++++++++++++
 packages/engine/src/worktree-pool.ts               |  2 +-
 14 files changed, 361 insertions(+), 43 deletions(-)

Fusion-Task-Id: FN-5671
Fusion-Task-Lineage: 7f2e7b68-050d-4a2c-af0b-cb895de35576

docs/dashboard-guide.md

@@ -111,6 +111,22 @@ Planning Mode now includes branch controls on the summary screen before you crea
 
 These values are sent with the Planning Mode create-task request as `branchSelection`, so created tasks persist branch/base-branch settings consistently with other branch-aware task creation flows.
 
+## New Task Modal Branch Strategy
+
+The **New Task** dialog uses the same four-option **Branch strategy** selector and `branchSelection` payload as Planning Mode:
+
+- `Use project/default branch`
+- `Create auto-named branch per task`
+- `Use existing branch`
+- `Create custom new branch`
+
+Rules:
+
+- `existing` and `custom-new` require a branch name.
+- `project-default` leaves `branch` unset.
+- `auto-new` creates a branch after task creation using `fusion/{task-id}-{short-name}` (for example `fusion/fn-5671-branch-strategy-dropdown`).
+- `Merge target / base branch` stays optional for all modes.
+
 ## Chat View
 
 Chat view provides project-scoped conversations with agents.