feat(KB-629): complete Step 6 — add documentation and changeset for bulk model editing

.changeset/add-bulk-model-edit.md

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+---
+"@gsxdsm/fusion": minor
+---
+
+Add bulk model editing to dashboard list view
+
+Users can now efficiently update AI model configuration for multiple tasks at once through the dashboard list view:
+
+- Select multiple tasks via checkboxes in the list view
+- New "Bulk Edit Models" toolbar appears when tasks are selected
+- Choose executor and/or validator models from dropdowns
+- Apply changes to all selected tasks in a single action
+- Selection persists in localStorage across page reloads
+- Archived tasks are excluded from bulk editing
+
+New API endpoint: `POST /api/tasks/batch-update-models`