feat(editor): add ReadonlyContent component for lightweight markdown display

- Add del selector to strikethrough CSS for react-markdown compatibility
- Create ReadonlyContent using react-markdown + lowlight + content-editor.css
- Export from editor module index

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/features/editor/content-editor.css

@@ -353,7 +353,8 @@
   font-style: italic;
 }
 
-.rich-text-editor s {
+.rich-text-editor s,
+.rich-text-editor del {
   text-decoration: line-through;
   color: var(--muted-foreground);
 }

apps/web/features/editor/index.ts

@@ -9,3 +9,4 @@ export {
   type TitleEditorRef,
 } from "./title-editor";
 export { copyMarkdown } from "./utils/clipboard";
+export { ReadonlyContent } from "./readonly-content";

apps/web/features/editor/readonly-content.tsx

@@ -0,0 +1,155 @@
+"use client";
+
+/**
+ * ReadonlyContent — lightweight markdown renderer for readonly content display.
+ *
+ * Replaces <ContentEditor editable={false}> for comment cards and other
+ * read-only surfaces. Uses react-markdown instead of a full Tiptap/ProseMirror
+ * instance, eliminating EditorView, Plugin, and NodeView overhead.
+ *
+ * Visual parity with ContentEditor is achieved by:
+ * - Wrapping output in <div class="rich-text-editor readonly"> so the same
+ *   content-editor.css rules apply to standard HTML tags
+ * - Using the same preprocessMarkdown pipeline (mention shortcodes + linkify)
+ * - Using lowlight for code highlighting (same engine as Tiptap's CodeBlockLowlight)
+ *   so .hljs-* CSS rules from content-editor.css produce identical colors
+ * - Rendering mentions with the same IssueMentionCard component and .mention class
+ */
+
+import { useMemo } from "react";
+import ReactMarkdown, {
+  defaultUrlTransform,
+  type Components,
+} from "react-markdown";
+import remarkGfm from "remark-gfm";
+import rehypeRaw from "rehype-raw";
+import { createLowlight, common } from "lowlight";
+import { toHtml } from "hast-util-to-html";
+import { cn } from "@/lib/utils";
+import { IssueMentionCard } from "@/features/issues/components/issue-mention-card";
+import { preprocessMarkdown } from "./utils/preprocess";
+import "./content-editor.css";
+
+// ---------------------------------------------------------------------------
+// Lowlight — same engine + language set as Tiptap's CodeBlockLowlight
+// ---------------------------------------------------------------------------
+
+const lowlight = createLowlight(common);
+
+// ---------------------------------------------------------------------------
+// URL transform — allow mention:// protocol through react-markdown's sanitizer
+// ---------------------------------------------------------------------------
+
+function urlTransform(url: string): string {
+  if (url.startsWith("mention://")) return url;
+  return defaultUrlTransform(url);
+}
+
+// ---------------------------------------------------------------------------
+// Custom react-markdown components
+// ---------------------------------------------------------------------------
+
+const components: Partial<Components> = {
+  // Links — route mention:// to mention components, others open in new tab
+  a: ({ href, children }) => {
+    if (href?.startsWith("mention://")) {
+      const match = href.match(
+        /^mention:\/\/(member|agent|issue|all)\/(.+)$/,
+      );
+      if (match?.[1] === "issue" && match[2]) {
+        const label =
+          typeof children === "string"
+            ? children
+            : Array.isArray(children)
+              ? children.join("")
+              : undefined;
+        return <IssueMentionCard issueId={match[2]} fallbackLabel={label} />;
+      }
+      // Member / agent / all mentions
+      return <span className="mention">{children}</span>;
+    }
+
+    // Regular links — open in new tab (matches ContentEditor readonly behavior)
+    return (
+      <a
+        href={href}
+        onClick={(e) => {
+          e.preventDefault();
+          if (href) window.open(href, "_blank", "noopener,noreferrer");
+        }}
+      >
+        {children}
+      </a>
+    );
+  },
+
+  // Tables — wrap in tableWrapper div for border/radius/scroll (matches Tiptap)
+  table: ({ children }) => (
+    <div className="tableWrapper">
+      <table>{children}</table>
+    </div>
+  ),
+
+  // Code — lowlight highlighting for blocks, plain render for inline
+  code: ({ className, children, node, ...props }) => {
+    const lang = /language-(\w+)/.exec(className || "")?.[1];
+    const isBlock =
+      node?.position &&
+      node.position.start.line !== node.position.end.line;
+
+    if (!isBlock && !lang) {
+      // Inline code — CSS handles styling via .rich-text-editor code
+      return <code {...props}>{children}</code>;
+    }
+
+    // Block code — highlight with lowlight, output hljs classes
+    const code = String(children).replace(/\n$/, "");
+    try {
+      const tree = lang
+        ? lowlight.highlight(lang, code)
+        : lowlight.highlightAuto(code);
+      return (
+        <code
+          className={cn("hljs", lang && `language-${lang}`)}
+          dangerouslySetInnerHTML={{ __html: toHtml(tree) }}
+        />
+      );
+    } catch {
+      // Fallback — render without highlighting
+      return (
+        <code className={className} {...props}>
+          {children}
+        </code>
+      );
+    }
+  },
+
+  // Pre — pass through (CSS handles styling via .rich-text-editor pre)
+  pre: ({ children }) => <pre>{children}</pre>,
+};
+
+// ---------------------------------------------------------------------------
+// Component
+// ---------------------------------------------------------------------------
+
+interface ReadonlyContentProps {
+  content: string;
+  className?: string;
+}
+
+export function ReadonlyContent({ content, className }: ReadonlyContentProps) {
+  const processed = useMemo(() => preprocessMarkdown(content), [content]);
+
+  return (
+    <div className={cn("rich-text-editor readonly text-sm", className)}>
+      <ReactMarkdown
+        remarkPlugins={[[remarkGfm, { singleTilde: false }]]}
+        rehypePlugins={[rehypeRaw]}
+        urlTransform={urlTransform}
+        components={components}
+      >
+        {processed}
+      </ReactMarkdown>
+    </div>
+  );
+}