Add protocol stubs for remote server communication infrastructure:
- ServerClient: Health check and version negotiation protocol
- RemoteExecutionClient: Remote plan execution and status protocol
- AuthClient: Authentication and token management protocol
- StubServerClient/StubRemoteExecutionClient/StubAuthClient: Stub
implementations raising NotImplementedError for all methods
- ServerConnectionConfig: Validated config model (server_url, namespace,
auth_token_ref, tls_verify)
- Config keys: core.server_url, core.server_namespace, core.server_tls_verify
- CLI: agents connect <url> stub command with explicit warning
- CLI: agents info shows Server Mode (disabled/stubbed)
ISSUES CLOSED: #201
