Add --stdin flag for eval command (#348)

Adds --stdin flag to read JavaScript from stdin, enabling heredoc usage
for multiline scripts without shell escaping issues.

cli/src/output.rs

@@ -885,6 +885,7 @@ Executes JavaScript code in the browser context and returns the result.
 
 Options:
   -b, --base64         Decode script from base64 (avoids shell escaping issues)
+  --stdin              Read script from stdin (useful for heredocs/multiline)
 
 Global Options:
   --json               Output as JSON
@@ -895,6 +896,12 @@ Examples:
   agent-browser eval "window.location.href"
   agent-browser eval "document.querySelectorAll('a').length"
   agent-browser eval -b "ZG9jdW1lbnQudGl0bGU="
+
+  # Read from stdin with heredoc
+  cat <<'EOF' | agent-browser eval --stdin
+  const links = document.querySelectorAll('a');
+  links.length;
+  EOF
 "##
         }