Rename setup CLI command to onboard for compatibility

Serde alias on `onboard_completed` preserves existing settings.json files
that still have the old `setup_completed` key.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

src/cli/tool.rs

@@ -717,7 +717,9 @@ async fn auth_tool(name: String, dir: Option<PathBuf>, user_id: String) -> anyho
     // Initialize secrets store
     let config = Config::from_env()?;
     let master_key = config.secrets.master_key().ok_or_else(|| {
-        anyhow::anyhow!("SECRETS_MASTER_KEY not set. Run 'ironclaw setup' first or set it in .env")
+        anyhow::anyhow!(
+            "SECRETS_MASTER_KEY not set. Run 'ironclaw onboard' first or set it in .env"
+        )
     })?;
 
     let store = Store::new(&config.database).await?;