FN-5883: restore planning-mode single-task history

Keep completed planning sessions restorable after single-task creation.

- preserve completed planning-session rows after create-task by releasing only in-memory runtime state
- keep completed sessions in the Planning Mode history UI and add coverage for history restoration flows
- document the restored Planning Mode history behavior and add a patch changeset for @runfusion/fusion

Files changed:
 .changeset/fn-5883-planning-restore.md             |   5 +
 docs/dashboard-guide.md                            |   2 +
 .../dashboard/app/components/PlanningModeModal.tsx |  17 +--
 .../PlanningModeModal.planning-flow.test.tsx       | 118 ++++++++++++++++++++-
 .../src/__tests__/routes-planning.test.ts          |  86 +++++++++++----
 packages/dashboard/src/planning.ts                 |   7 +-
 .../src/routes/register-planning-subtask-routes.ts |  14 +--
 7 files changed, 207 insertions(+), 42 deletions(-)

Fusion-Task-Id: FN-5883

Fusion-Task-Lineage: 47824bac-3083-45a4-a0d5-b8c7540966c1

docs/dashboard-guide.md

@@ -113,6 +113,8 @@ 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.
 
+Completed single-task planning sessions remain in the Planning Mode history after you create the task, and selecting one restores the completed summary instead of restarting the composer.
+
 ## New Task Modal Branch Strategy
 
 The **New Task** dialog uses the same four-option **Branch strategy** selector and `branchSelection` payload as Planning Mode: