feat(web): add agent message comments (#1835)

* feat(web): add agent message comments

* fix(web): harden agent message comments

* fix(web): address message comment review findings

* fix(web): allow comments on legacy agent messages

* fix(web): address latest message comment review

* fix(web): address follow-up message comment review

* fix(web): address remaining message comment review

* fix(web): keep failed mobile comments open

* fix(web): preserve selections over message comments

apps/web/components/task/chat/__evals__/render-cost.test.tsx

@@ -23,6 +23,10 @@ vi.mock("@/components/shared/markdown-components", () => ({
 
 import { MemoizedMarkdown } from "@/components/shared/memoized-markdown";
 import { __markdownParseCount, __resetMarkdownCounters } from "@/lib/markdown/normalize-cache";
+import {
+  createMessageTextAnchor,
+  getMessageCommentDecorations,
+} from "@/lib/chat/agent-message-comments";
 
 type Row = { id: string; content: string };
 
@@ -121,4 +125,30 @@ describe("chat render-cost evals", () => {
     render(<MessageList rows={rows} panel={false} />);
     expect(__markdownParseCount()).toBe(1);
   });
+
+  it("scenario 5: restoring many inline highlights does not reparse markdown rows", () => {
+    const content = Array.from({ length: 20 }, (_, index) => `reply-${index}`).join(" ");
+    const root = document.createElement("div");
+    root.textContent = content;
+    const comments = Array.from({ length: 20 }, (_, index) => {
+      const selectedText = `reply-${index}`;
+      const start = content.indexOf(selectedText);
+      return {
+        id: `comment-${index}`,
+        sessionId: "session-1",
+        source: "agent-message" as const,
+        messageId: "reply-1",
+        selectedText,
+        text: "feedback",
+        createdAt: "2026-07-20T00:00:00Z",
+        status: "pending" as const,
+        anchor: createMessageTextAnchor("reply-1", content, start, start + selectedText.length),
+      };
+    });
+
+    const parsesBefore = __markdownParseCount();
+    expect(getMessageCommentDecorations(root, comments)).toHaveLength(20);
+    expect(root.querySelectorAll("mark[data-agent-message-comment-id]")).toHaveLength(0);
+    expect(__markdownParseCount() - parsesBefore).toBe(0);
+  });
 });