refactor CLI to use shared clients from context

- Add Login() method to toughswitch and edgeos clients
- Use zerolog's built-in context methods for logger storage
- Add context helpers for toughswitch/edgeos clients
- Create prepareClients prerun to initialize clients from config
- Consolidate device fetching into shared client.go helper

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/toughswitch/client.go

@@ -120,6 +120,17 @@ func (c *Client) getDeviceByHost(host string) (*deviceClient, error) {
 	return d, nil
 }
 
+// Login authenticates with the device at the given host.
+// This is called automatically on 401 responses, but can be called explicitly
+// to pre-authenticate before making requests.
+func (c *Client) Login(ctx context.Context, host string) error {
+	d, err := c.getDeviceByHost(host)
+	if err != nil {
+		return err
+	}
+	return d.login(ctx)
+}
+
 func (d *deviceClient) login(ctx context.Context) error {
 	d.mu.Lock()
 	defer d.mu.Unlock()