feat: CodeMirror markdown editor for identity/memory file editor

Replace plain textarea with CodeMirror 5 + markdown mode loaded from
jsDelivr CDN. Editor fills the modal body via flex layout, theme-aware
via CSS vars (cursor, selection, headings, bold/em/links/code all mapped
to Cortex dark/light palette). Lazy init on first file open; history
cleared per-file so undo doesn't bleed across files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/app.js

@@ -1255,7 +1255,7 @@
         // ── File editor ──────────────────────────────────────────────
         const fileModal       = document.getElementById('file-modal');
         const fileSidebar     = document.getElementById('file-sidebar');
-        const fileEditor     = document.getElementById('file-editor');
+        const fileEditorWrap  = document.getElementById('file-editor-wrap');
         const filePreview     = document.getElementById('file-preview');
         const fileRawBtn      = document.getElementById('file-raw-btn');
         const filePreviewBtn  = document.getElementById('file-preview-btn');
@@ -1265,6 +1265,20 @@
 
         let fileMode       = 'preview'; // 'edit' or 'preview'
         let activeFileName = null;
+        let mdEditor       = null;
+
+        function initMdEditor() {
+            if (mdEditor) return;
+            mdEditor = CodeMirror(fileEditorWrap, {
+                mode: 'markdown',
+                lineWrapping: true,
+                lineNumbers: false,
+                autofocus: false,
+                tabSize: 2,
+                indentWithTabs: false,
+                extraKeys: { 'Ctrl-S': () => { fileSaveBtn.click(); return false; } },
+            });
+        }
 
         // File groups — controls sidebar order and section labels
         const FILE_GROUPS = [
@@ -1346,17 +1360,19 @@
         function setFileMode(mode) {
             fileMode = mode;
             if (mode === 'edit') {
-                fileEditor.classList.remove('hidden');
+                fileEditorWrap.classList.remove('hidden');
                 filePreview.classList.remove('active');
                 fileRawBtn.classList.add('active');
                 filePreviewBtn.classList.remove('active');
+                mdEditor.refresh();
+                mdEditor.focus();
             } else {
-                fileEditor.classList.add('hidden');
+                fileEditorWrap.classList.add('hidden');
                 filePreview.classList.add('active');
                 fileRawBtn.classList.remove('active');
                 filePreviewBtn.classList.add('active');
                 if (typeof marked !== 'undefined') {
-                    filePreview.innerHTML = marked.parse(fileEditor.value);
+                    filePreview.innerHTML = marked.parse(mdEditor.getValue());
                     filePreview.querySelectorAll('a').forEach(a => {
                         a.target = '_blank'; a.rel = 'noopener noreferrer';
                     });
@@ -1366,10 +1382,12 @@
 
         async function loadFile(name) {
             setActiveFile(name);
+            initMdEditor();
             const res = await fetch(`/files/${encodeURIComponent(name)}?${_fileParams}`);
-            if (!res.ok) { fileEditor.value = `Error loading ${name}`; return; }
+            if (!res.ok) { mdEditor.setValue(`Error loading ${name}`); return; }
             const data = await res.json();
-            fileEditor.value = data.content;
+            mdEditor.setValue(data.content);
+            mdEditor.clearHistory();
             setFileMode(fileMode);
         }
 
@@ -1396,7 +1414,7 @@
             const res = await fetch(`/files/${encodeURIComponent(activeFileName)}?${_fileParams}`, {
                 method: 'PUT',
                 headers: { 'Content-Type': 'application/json' },
-                body: JSON.stringify({ content: fileEditor.value }),
+                body: JSON.stringify({ content: mdEditor.getValue() }),
             });
             if (res.ok) {
                 showToast('File saved', 'success');
@@ -1421,13 +1439,13 @@
         const sessionSearchResults = document.getElementById('session-search-results');
 
         function _showFileView() {
-            fileEditor.style.display = '';
+            setFileMode(fileMode);
             filePreview.style.display = '';
             sessionSearchResults.style.display = 'none';
         }
 
         function _showSearchResults(html) {
-            fileEditor.style.display = 'none';
+            fileEditorWrap.classList.add('hidden');
             filePreview.style.display = 'none';
             sessionSearchResults.style.display = '';
             sessionSearchResults.innerHTML = html;

cortex/static/index.html

@@ -20,6 +20,9 @@
         })();
     </script>
     <link rel="stylesheet" href="/static/style.css">
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/codemirror@5.65.17/lib/codemirror.min.css">
+    <script src="https://cdn.jsdelivr.net/npm/codemirror@5.65.17/lib/codemirror.min.js"></script>
+    <script src="https://cdn.jsdelivr.net/npm/codemirror@5.65.17/mode/markdown/markdown.min.js"></script>
     <script src="/static/marked.min.js"></script>
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.11.1/styles/atom-one-dark.min.css">
     <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.11.1/highlight.min.js"></script>
@@ -140,7 +143,7 @@
                     </div>
                 </div>
                 <div id="file-modal-body">
-                    <textarea id="file-editor" spellcheck="false"></textarea>
+                    <div id="file-editor-wrap"></div>
                     <div id="file-preview"></div>
                     <div id="session-search-results" style="display:none"></div>
                 </div>

cortex/static/style.css

@@ -1141,20 +1141,43 @@
             flex-direction: column;
         }
 
-        #file-editor {
+        /* CodeMirror markdown editor */
+        #file-editor-wrap {
             flex: 1;
-            width: 100%;
+            min-height: 0;
+            overflow: hidden;
+            display: flex;
+            flex-direction: column;
+        }
+        #file-editor-wrap.hidden { display: none; }
+
+        #file-editor-wrap .CodeMirror {
+            flex: 1;
+            height: 100%;
             background: var(--bg);
             color: var(--text);
-            border: none;
-            outline: none;
-            padding: 16px;
             font-family: 'Courier New', monospace;
             font-size: 0.85rem;
             line-height: 1.55;
-            resize: none;
+            border: none;
-            display: block;
         }
+        #file-editor-wrap .CodeMirror-scroll { padding: 12px 16px; }
+        #file-editor-wrap .CodeMirror-lines  { padding: 0; }
+        #file-editor-wrap .CodeMirror-cursor { border-left-color: var(--accent); }
+        #file-editor-wrap .CodeMirror-selectedtext { background: var(--border) !important; }
+        #file-editor-wrap .CodeMirror-selected { background: var(--border) !important; }
+        #file-editor-wrap .CodeMirror-focused .CodeMirror-selected { background: var(--border) !important; }
+
+        /* Markdown token colours */
+        #file-editor-wrap .cm-header   { color: var(--accent); font-weight: 600; }
+        #file-editor-wrap .cm-strong   { color: var(--text); font-weight: 700; }
+        #file-editor-wrap .cm-em       { color: var(--muted); font-style: italic; }
+        #file-editor-wrap .cm-link     { color: #a78bfa; }
+        #file-editor-wrap .cm-url      { color: var(--muted); }
+        #file-editor-wrap .cm-comment  { color: var(--muted); }
+        #file-editor-wrap .cm-quote    { color: var(--muted); font-style: italic; }
+        #file-editor-wrap .cm-code     { color: var(--muted); background: var(--surface); }
+        #file-editor-wrap .cm-hr       { color: var(--border); }
 
         #file-preview {
             flex: 1;
@@ -1165,7 +1188,7 @@
         }
 
         #file-preview.active      { display: block; }
-        #file-editor.hidden  { display: none; }
+        #file-editor-wrap.hidden  { display: none; }
 
         /* Talk activity badge on Sessions button */
         #sessions-btn.talk-badge::after {