fix(migration): correct schema mismatch between migration script and Document model

SUMMARY:
Fixed visibility field schema mismatch causing documents to be internal
when they should be public.

ISSUE:
Migration script used 'public: true' (boolean) but Document model requires
'visibility: "public"' (string enum). Documents migrated with incorrect
schema weren't appearing in public document lists.

FIX:
- Changed migration script to use visibility field (string)
- Map legacy 'public' boolean to 'visibility' string
- Valid values: 'public', 'internal', 'confidential', 'archived'
- Also extract 'category' from frontmatter (was using metadata.type)

VALIDATION:
- Research governance ROI case study now appears with visibility: public
- Category: case-studies (for docs.html sidebar)
- Frontmatter properly parsed and mapped to schema

RELATED:
Enables proper publication of research case study to public documentation.

🤖 Generated with Claude Code (https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

scripts/migrate-documents.js

@@ -167,10 +167,29 @@ async function processMarkdownFile(filePath, sourcePath) {
     slug.toLowerCase().includes(pattern)
   );
 
-  // Check front matter for explicit public field
-  const isPublic = frontMatter.public !== undefined
-    ? frontMatter.public === true || frontMatter.public === 'true'
-    : !isInternal; // Default to public unless it matches internal patterns
+  // Determine visibility from front matter or default based on patterns
+  let visibility = frontMatter.visibility || null;
+
+  if (!visibility) {
+    // Check for legacy public field
+    if (frontMatter.public !== undefined) {
+      const isPublic = frontMatter.public === true || frontMatter.public === 'true';
+      visibility = isPublic ? 'public' : 'internal';
+    } else {
+      // Default to internal if matches internal patterns, otherwise public
+      visibility = isInternal ? 'internal' : 'public';
+    }
+  }
+
+  // Validate visibility value
+  const validVisibility = ['public', 'internal', 'confidential', 'archived'];
+  if (!validVisibility.includes(visibility)) {
+    console.warn(`Invalid visibility '${visibility}' for ${filename}, defaulting to 'internal'`);
+    visibility = 'internal';
+  }
+
+  // Determine category from front matter or metadata type
+  const category = frontMatter.category || metadata.type || 'none';
 
   // Build document object matching Document model schema
   const doc = {
@@ -178,10 +197,11 @@ async function processMarkdownFile(filePath, sourcePath) {
     slug: slug,
     quadrant: metadata.quadrant,
     persistence: 'HIGH', // Default for technical documents
+    visibility: visibility,
+    category: category,
     content_html: htmlContent,
     content_markdown: content,
     toc: tableOfContents,
-    public: isPublic,
     metadata: {
       author: metadata.author,
       version: metadata.version,