Update prompt input placeholder to mention paste support

- Remove /command reference as commands are not currently supported
- Add mention of paste support for images and text

src/components/prompt-input.tsx

@@ -711,7 +711,7 @@ export default function PromptInput(props: PromptInputProps) {
           <textarea
             ref={textareaRef}
             class="prompt-input"
-            placeholder="Type your message, @file, @agent, or /command..."
+            placeholder="Type your message, @file, @agent, or paste images and text..."
             value={prompt()}
             onInput={handleInput}
             onKeyDown={handleKeyDown}