fix: add manual Fern Agent install instructions + video

fern/products/docs/pages/ai/fern-agent.mdx

@@ -158,7 +158,30 @@ Open the [Fern Dashboard](https://dashboard.buildwithfern.com/) and start a conv
 * **Connect Fern Agent to Slack.** Click **Connect Slack**, select the workspace, and add Fern to the Slack channels where your team discusses documentation. Authorizing the app requires Slack admin access.
 * **Connect your agents.** Click **Onboard your agent** to connect your coding agent to [Fern's MCP server](/learn/docs/ai-features/fern-mcp-servers).
 
-Answering reader questions in Slack is a separate capability with its own install, covered on the [Ask Fern Slack app](/learn/docs/ai-features/ask-fern/slack-app) page.
+<Frame>
+  <video
+    src="./assets/agent-install.mp4"
+    autoPlay
+    loop
+    playsInline
+    muted
+  >
+  </video>
+</Frame>
+
+Alternatively, install manually. Connect Slack with your [Fern API key](/learn/dashboard/configuration/api-keys) and the `organization` field from `fern/fern.config.json`:
+
+```bash
+curl -X POST https://fai.buildwithfern.com/slack/agent/install \
+  -H "Authorization: Bearer <FERN_TOKEN>" -H "Content-Type: application/json" \
+  -d '{"org_id":"<org>"}'
+```
+
+Onboard your agent by handing it this prompt:
+
+<Prompt title="Set up Fern Agent in your terminal">
+Fetch and execute the appropriate instructions to set me up for Fern from https://buildwithfern.com/learn/docs/ai-features/agent-setup.md
+</Prompt>
 
 ## Making requests in Slack