feat: add markdown import/export (closes #77, #78)

- Add POST /api/knowledge/import endpoint for .md and .zip files
- Add GET /api/knowledge/export endpoint with markdown/json formats
- Import parses frontmatter (title, tags, status, visibility)
- Export includes frontmatter in markdown format
- Add ImportExportActions component with drag-and-drop UI
- Add import progress dialog with success/error summary
- Add export dropdown with format selection
- Include comprehensive test suite
- Support bulk import with detailed error reporting

apps/web/src/app/(authenticated)/knowledge/page.tsx

@@ -4,6 +4,7 @@ import { useState, useMemo } from "react";
 import { EntryStatus } from "@mosaic/shared";
 import { EntryList } from "@/components/knowledge/EntryList";
 import { EntryFilters } from "@/components/knowledge/EntryFilters";
+import { ImportExportActions } from "@/components/knowledge";
 import { mockEntries, mockTags } from "@/lib/api/knowledge";
 import Link from "next/link";
 import { Plus } from "lucide-react";
@@ -99,22 +100,35 @@ export default function KnowledgePage() {
   return (
     <main className="container mx-auto px-4 py-8 max-w-5xl">
       {/* Header */}
-      <div className="mb-8 flex items-center justify-between">
-        <div>
-          <h1 className="text-3xl font-bold text-gray-900">Knowledge Base</h1>
-          <p className="text-gray-600 mt-2">
-            Documentation, guides, and knowledge entries
-          </p>
+      <div className="mb-8">
+        <div className="flex items-center justify-between mb-4">
+          <div>
+            <h1 className="text-3xl font-bold text-gray-900">Knowledge Base</h1>
+            <p className="text-gray-600 mt-2">
+              Documentation, guides, and knowledge entries
+            </p>
+          </div>
+
+          {/* Create button */}
+          <Link
+            href="/knowledge/new"
+            className="inline-flex items-center gap-2 px-4 py-2 bg-blue-600 text-white rounded-lg hover:bg-blue-700 transition-colors shadow-sm"
+          >
+            <Plus className="w-5 h-5" />
+            <span>Create Entry</span>
+          </Link>
         </div>
 
-        {/* Create button */}
-        <Link
-          href="/knowledge/new"
-          className="inline-flex items-center gap-2 px-4 py-2 bg-blue-600 text-white rounded-lg hover:bg-blue-700 transition-colors shadow-sm"
-        >
-          <Plus className="w-5 h-5" />
-          <span>Create Entry</span>
-        </Link>
+        {/* Import/Export Actions */}
+        <div className="flex justify-end">
+          <ImportExportActions
+            onImportComplete={() => {
+              // TODO: Refresh the entry list when real API is connected
+              // For now, this would trigger a refetch of the entries
+              window.location.reload();
+            }}
+          />
+        </div>
       </div>
 
       {/* Filters */}