fix(mcp): say not to open the LinkedIn callback URL

The highlighted box added in fa86fc5 made the URL prominent and
copyable, and an operator promptly copied it and opened it in a browser
tab — a reasonable reading of "here is a URL, copy it" that lands on a
page which cannot do anything without a login attached.

Reworded to lead with the verb that matters ("paste this into ... then
save it there") and added the missing half in plain words: don't open it
yourself, it is where LinkedIn sends you back after login. The backend
callback now explains the same thing for anyone who opens it anyway.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/pages/Agent/MCP/index.tsx

@@ -492,9 +492,20 @@ function MCPPage() {
                     color: "#92400e",
                   }}
                 >
-                  Before clicking Authenticate: add this exact URL to this
+                  Before clicking Authenticate: paste this exact URL into
-                  LinkedIn app's Authorized redirect URLs (Developer Portal
+                  LinkedIn Developer Portal → your app → Auth tab →
-                  → your app → Auth tab), or LinkedIn will refuse the login.
+                  Authorized redirect URLs, then save it there. LinkedIn
+                  refuses the login until it matches.
+                </p>
+                <p
+                  style={{
+                    margin: "4px 0 0",
+                    fontSize: 12,
+                    color: "#92400e",
+                  }}
+                >
+                  Don't open this URL yourself — it is where LinkedIn sends
+                  you back after login, not a page to visit.
                 </p>
                 <div
                   style={{