docs: document @-mentioning dashboard tiles in AI agent threads

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

@@ -41,6 +41,17 @@ This opens a new tab on the new-thread page for your default agent. The chart or
 - Click the pinned card to open the chart or dashboard in a new tab.
 - The pinned context persists across follow-up messages — *"now break it down by region"* still refers to the originally pinned chart.
 
+### Mentioning dashboard tiles
+
+When the pinned context is a dashboard, you can `@`-mention any of its chart tiles directly in your message to focus the agent on a single tile.
+
+Type `@` in the input and pick a tile from the **Dashboard tiles** group in the suggestion list. The selected chart is added alongside the pinned dashboard, so the agent answers about that specific tile while still keeping the dashboard as overall context.
+
+This is useful when a dashboard has many charts and you want to drill into one without leaving the thread:
+
+- *"Why did @Weekly active users drop last week?"*
+- *"Break @Revenue by region down by channel."*
+
 ### What you can ask
 
 When you pin a saved chart, the agent can read its actual data (subject to your [data access setting](/guides/ai-agents/data-access)). It honors the chart's saved filters, sorts, and custom metrics, so you can ask: