feat: 'Tell the AI more' free-text brief for AI names (v0.5.0)

Appended to the shared prompt (lib/ai-prompt.ts) on both AI paths; capped at
300 chars client- and server-side; shown only when AI is on; kept in
sessionStorage so it survives 'Show me more'.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XStQKxPfEjrTvWo83KFCxG

src/app/globals.css

@@ -398,6 +398,10 @@ h2 {
   text-decoration: underline;
 }
 
+.field.brief {
+  margin-top: 4px;
+}
+
 .signin {
   font-size: 14px;
   text-decoration: none;