docs: document @ mentions for content in AI chat input

guides/ai-agents/using-ai-agents.mdx

@@ -51,6 +51,23 @@ When you pin a saved chart, the agent can read its actual data (subject to your
 
 If your agent has data access disabled, pinning still works — the agent sees the chart's structure (name, dimensions, metrics) but no row values are sent to the underlying LLM.
 
+## Mentioning content with `@`
+
+You can pull charts and dashboards into a conversation directly from the chat input by typing `@` followed by part of the name. A suggestion menu appears with matching content you have access to, grouped to help you find the right item quickly:
+
+- **Already mentioned** — content referenced earlier in this thread
+- **Current page** — the chart or dashboard you're viewing when you open the agent
+- **Dashboard tiles** — charts on the dashboard you're viewing
+- **Search results** — anything else you can access in the project
+
+Select an item with the arrow keys and press <kbd>Enter</kbd>, or click it. The mention appears as a chip in your message, and the agent receives the selected chart or dashboard as context when you send.
+
+<Tip>
+  While the suggestion menu is open, <kbd>Enter</kbd> selects a mention instead of submitting the message. Press <kbd>Esc</kbd> to close the menu, then <kbd>Enter</kbd> to send.
+</Tip>
+
+Mentions are scoped to content you can already access — the suggestions only include charts and dashboards your role and space permissions allow you to view. You can mention multiple items in a single message (for example, *"compare @Q3 revenue with @Q4 revenue"*), and the referenced content is carried through follow-up messages in the same thread.
+
 ## Read-only threads
 
 In some cases, the chat input on an existing thread is disabled and you can't send follow-up messages. The input shows a short message explaining why. You'll see this when: