agent: load external stylesheets by default with LLM

Enables `load_external_stylesheets` when an LLM client is active,
as LLM drivers reason about visibility and computed styles.
Updates the help documentation to reflect this change.

src/agent/Agent.zig

@@ -277,6 +277,14 @@ pub fn init(allocator: std.mem.Allocator, app: *App, opts: Config.Agent) !*Agent
     self.ai_client = if (llm) |l| try initAiClient(allocator, l, opts.base_url) else null;
     errdefer if (self.ai_client) |c| deinitAiClient(allocator, c);
 
+    // An LLM driver reasons about visibility/computed styles, so fetch external
+    // stylesheets by default. Pure replay and --no-llm keep the cheap fast path.
+    // The --enable-external-stylesheets flag is already folded into the session
+    // default, so this only ever turns the feature on.
+    if (self.ai_client != null) {
+        self.session.load_external_stylesheets = true;
+    }
+
     if (will_repl) {
         self.terminal.attachCompleter();
         self.terminal.completion_source = .{

src/help.zon

@@ -255,7 +255,7 @@
     \\          Fetch external <link rel=stylesheet> resources so their rules
     \\          contribute to computed styles (and therefore to visibility checks like
     \\          display, visibility, opacity, pointer-events).
-    \\          Defaults to false.
+    \\          Defaults to false, except in agent mode with an LLM, where it is on.
     \\  --block-private-networks
     \\          Block HTTP requests to private/internal IP addresses after DNS
     \\          resolution.