fix(ideas): respecteer YAML-volgorde bij plan-materialize

Tasks erven nu story-priority i.p.v. eigen task.priority bij
materializeIdeaPlanAction. Worker sorteert op `priority ASC, sort_order ASC`;
gemixte task-priorities binnen één story zouden anders de YAML-volgorde
verstoren (e.g. tasks met priority 1/1/1/2/1/2 → worker-volgorde 1,2,3,5,4,6
i.p.v. 1,2,3,4,5,6).

- actions/ideas.ts: priority = s.priority bij task-create
- lib/schemas/idea.ts: task.priority optional (geaccepteerd, genegeerd)
- lib/idea-prompts/make-plan.md: documenteer dat task.priority genegeerd wordt
- __tests__/lib/idea-schemas.test.ts: test dat omitted task.priority slaagt

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

__tests__/lib/idea-schemas.test.ts

@@ -128,4 +128,21 @@ describe('ideaPlanMdFrontmatterSchema', () => {
     })
     expect(r.success).toBe(false)
   })
+
+  it('accepts plan with task.priority omitted (inherits story-priority via materialize)', () => {
+    const r = ideaPlanMdFrontmatterSchema.safeParse({
+      ...validPlan,
+      stories: [
+        {
+          title: 'Story zonder task-priorities',
+          priority: 2,
+          tasks: [
+            { title: 'Taak 1' }, // geen priority — moet geaccepteerd
+            { title: 'Taak 2', verify_required: 'ALIGNED' },
+          ],
+        },
+      ],
+    })
+    expect(r.success).toBe(true)
+  })
 })