feat(editor): unify Tiptap editor for editing and readonly display

Replace react-markdown in comment/reply display with Tiptap readonly mode,
ensuring visual consistency between editing and viewing. Extract shared
BaseMentionExtension with MentionView NodeView used in both modes — issue
mentions render as inline cards with StatusIcon, clickable to open in new tab.
Redesign mention suggestion popup with grouped sections (Users/Issues),
agent badges, and StatusIcon for issues.

- New: mention-extension.ts (shared mention core)
- New: mention-view.tsx (shared NodeView for both modes)
- New: readonly-editor.tsx (lightweight Tiptap readonly wrapper)
- Modified: rich-text-editor.tsx (import from shared mention-extension)
- Modified: rich-text-editor.css (readonly + issue-mention overrides)
- Modified: comment-card.tsx (Markdown → ReadonlyEditor)
- Modified: mention-suggestion.tsx (grouped UI, StatusIcon, agent badge)

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

apps/web/features/issues/components/comment-card.tsx

@@ -21,7 +21,7 @@ import { cn } from "@/lib/utils";
 import { useActorName } from "@/features/workspace";
 import { timeAgo } from "@/shared/utils";
 import { RichTextEditor, type RichTextEditorRef } from "@/components/common/rich-text-editor";
-import { Markdown } from "@/components/markdown/Markdown";
+import { ReadonlyEditor } from "@/components/common/readonly-editor";
 import { FileUploadButton } from "@/components/common/file-upload-button";
 import { useFileUpload } from "@/shared/hooks/use-file-upload";
 import { ReplyInput } from "./reply-input";
@@ -193,7 +193,7 @@ function CommentRow({
       ) : (
         <>
           <div className="mt-1.5 pl-8 text-sm leading-relaxed text-foreground/85">
-            <Markdown mode="minimal">{entry.content ?? ""}</Markdown>
+            <ReadonlyEditor content={entry.content ?? ""} />
           </div>
           {!isTemp && (
             <ReactionBar
@@ -393,7 +393,7 @@ function CommentCard({
             ) : (
               <>
                 <div className="pl-10 text-sm leading-relaxed text-foreground/85">
-                  <Markdown mode="minimal">{entry.content ?? ""}</Markdown>
+                  <ReadonlyEditor content={entry.content ?? ""} />
                 </div>
                 {!isTemp && (
                   <ReactionBar