docs: Lead SSH key registration with commit signing

The SSH key registration section introduced setup before explaining its purpose.
Lead with signing Git commits and keep the matching GitHub key setup in the same section.

content/manuals/agentic-platform/sandboxes.md

@@ -82,6 +82,10 @@ in the panel for your chosen connection method.
 
 ### Register an SSH key
 
+To sign Git commits in Docker Agentic Platform, register your public SSH key
+in the Console and add the same key to GitHub as a signing key. You can also
+use the registered key to connect to your sandbox over SSH.
+
 1. Open the user menu and select **Settings**.
 2. Under **Sandbox SSH key**, optionally enter a **Name** for the key.
 3. Paste your public key into **Public key**, then select **Add key**. Use the
@@ -92,8 +96,8 @@ Only the public key is stored. It takes effect immediately, without restarting
 a sandbox. Check **Registered key** to review it. Adding another key replaces
 the registered key. To remove it, select **Delete key** and confirm deletion.
 
-To use the registered key to sign Git commits in Docker Agentic Platform,
-add the same public key to your GitHub account as a signing key:
+After registering the key, add the same public key to your GitHub account
+as a signing key:
 
 1. In your GitHub account settings, open **SSH and GPG keys** and select
    **New SSH key**.