FN-6137: convert PR lifecycle built-in to fragment

Convert the built-in PR lifecycle into a reusable fragment instead of a selectable task workflow.

- mark builtin:pr-workflow as a fragment and exclude fragments from default-enabled built-ins
- hide built-in fragments from the General settings workflow toggles
- update core and dashboard tests to assert fragment behavior and reject task selection of the PR lifecycle fragment
- add a changeset for the published CLI package

Files changed:
 .changeset/pr-lifecycle-fragment.md                |  5 +++++
 .../core/src/__tests__/builtin-workflows.test.ts   | 24 ++++++++++++++++------
 packages/core/src/builtin-workflows.ts             |  7 ++++---
 packages/core/src/store.ts                         |  2 +-
 .../__tests__/WorkflowNodeEditor.test.tsx          |  3 ++-
 .../settings/sections/GeneralSection.tsx           |  4 +++-
 6 files changed, 33 insertions(+), 12 deletions(-)

Fusion-Task-Id: FN-6137

Fusion-Task-Lineage: d959ba3b-2e50-4c9b-a1fa-161776939403

.changeset/pr-lifecycle-fragment.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": minor
+---
+
+Convert the built-in PR lifecycle from a selectable task workflow into a reusable workflow-editor fragment template.