FN-5717: add per-task auto-merge override in new task form

Add a New Task form option to override auto-merge behavior per task and wire it through task creation APIs.

- add TaskForm UI state/prop support for enabling or disabling per-task auto-merge override
- pass override values through NewTaskModal and legacy task creation API payloads
- persist and validate settings.autoMerge overrides in task workflow route handling
- add dashboard and route tests covering override defaults and explicit true/false behavior
- document per-task auto-merge override behavior in dashboard and settings docs

Files changed:
 docs/dashboard-guide.md                            |  1 +
 docs/settings-reference.md                         |  2 +-
 packages/dashboard/app/api/legacy.ts               |  2 +
 packages/dashboard/app/components/NewTaskModal.tsx | 11 ++++-
 packages/dashboard/app/components/TaskForm.tsx     | 27 ++++++++++++
 .../app/components/__tests__/NewTaskModal.test.tsx | 51 ++++++++++++++++++++++
 .../dashboard/src/__tests__/routes-tasks.test.ts   | 33 ++++++++++++++
 .../src/routes/register-task-workflow-routes.ts    |  6 +++
 8 files changed, 130 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-5717

Fusion-Task-Lineage: 3bdbb561-9c6d-4f64-ad4b-ae723e73223e

packages/dashboard/app/api/legacy.ts

@@ -377,6 +377,7 @@ export async function createTask(
     summarize,
     reviewLevel,
     executionMode,
+    autoMerge,
     priority,
     source,
     nodeId,
@@ -412,6 +413,7 @@ export async function createTask(
       summarize,
       reviewLevel,
       executionMode,
+      autoMerge,
       priority,
       source,
       nodeId,