refactor(markdown-editor): drop dead export + trim duplicated comment

- FaithfulMarkdownText: drop the `export` — it has zero importers, used only
  inline by createMarkdownLayer in the same file (its sibling createFaithfulMarked
  is already a private const).
- markdown-editor.tsx: remove the reset-history tail comment that restated verbatim
  the invariant already in resetUndoHistory's header (lines 52-53); the 'when it
  fires' explanation above it stays.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/shared/markdown-editor/markdown-editor-marked.ts

@@ -64,7 +64,7 @@ const createFaithfulMarked = () => {
 // per-extension hook), so replace that one method with identity, keeping load and save byte-symmetric.
 type ManagerWithEncode = { encodeTextForMarkdown: (text: string) => string };
 
-export const FaithfulMarkdownText = Extension.create({
+const FaithfulMarkdownText = Extension.create({
     name: 'faithfulMarkdownText',
     onBeforeCreate() {
         const manager = this.editor.markdown as unknown as ManagerWithEncode | undefined;

frontend/src/components/shared/markdown-editor/markdown-editor.tsx

@@ -234,10 +234,6 @@ function MarkdownEditor({
         //   - on every external `setContent` (e.g. parent calls
         //     form.reset(serverDocument)), because that transaction is
         //     not a user edit either.
-        //
-        // We MUST NOT reset on every render: when the user types,
-        // `value` changes too, and resetting then would erase the
-        // user's own undo history.
         if (!hasResetInitialHistoryRef.current || shouldExternalSync) {
             hasResetInitialHistoryRef.current = true;
             resetUndoHistory(editor);