CLI: Update Go SDK to 8dc9875576ae and add credentials + proxy check commands

## SDK Update
- Updated kernel-go-sdk to 8dc9875576aea5daa08fa44faeee0bd0e99888a8

## New Commands
- `kernel credentials list` - List stored credentials
- `kernel credentials get <id-or-name>` - Get credential details
- `kernel credentials create` - Create a new credential with name, domain, values
- `kernel credentials update <id-or-name>` - Update credential properties
- `kernel credentials delete <id-or-name>` - Delete a credential
- `kernel credentials totp-code <id-or-name>` - Get current TOTP code for 2FA

- `kernel proxies check <id>` - Check proxy health status

## Coverage Analysis
Full enumeration of SDK methods (api.md) vs CLI commands performed.

Author: Cursor Agent