ST-cmowjewfg: Test: parser geeft hint bij markdown-in-frontmatter

Voeg twee Vitest-cases toe:
- hints when markdown sneaks into frontmatter: fixture met [unclosed op
  een genummerde markdown-regel triggert YAMLParseError op die regel
  (plain lijst zonder unclosed flow parset als geldig YAML)
- omits hint for non-markdown yaml errors: unclosed bracket zonder
  markdown-patroon geeft geen hint

__tests__/lib/idea-plan-parser.test.ts

@@ -62,6 +62,41 @@ body
     }
   })
 
+  it('hints when markdown sneaks into frontmatter', () => {
+    // "1. **...**: [unclosed" triggers a YAMLParseError at the markdown line
+    // (plain-list-with-bold parses as valid YAML without an unclosed flow)
+    const broken = `---
+pbi:
+  title: Test
+  priority: 2
+stories:
+1. **Toggle zichtbaar in productie**: [unclosed
+---
+
+body
+`
+    const r = parsePlanMd(broken)
+    expect(r.ok).toBe(false)
+    if (!r.ok) {
+      expect(r.errors[0].hint).toMatch(/markdown/i)
+      expect(r.errors[0].line).toBeGreaterThan(1)
+    }
+  })
+
+  it('omits hint for non-markdown yaml errors', () => {
+    const broken = `---
+pbi:
+  title: Test
+  priority: [unclosed
+stories:
+  - foo
+---
+`
+    const r = parsePlanMd(broken)
+    expect(r.ok).toBe(false)
+    if (!r.ok) expect(r.errors[0].hint).toBeUndefined()
+  })
+
   it('reports schema-validation error when pbi-section missing', () => {
     const noPbi = `---
 stories: