Add README guidance for migrating to MXC Sessions

Problem

Users with an existing WSL-based OpenClaw Gateway do not have clear README guidance for moving to the native Windows Gateway running inside an MXC Session. The supported migration path, prerequisites, preservation guarantees, and recovery steps need to be explicit before MXC Sessions become the primary local setup model.

Proposed documentation

Add a migration section to the repository README that explains:

• What an MXC Session is and why the local Gateway runs inside one.
• Which Windows and MXC capabilities are required.
• How to determine whether the current PC supports the migration.
• How to migrate an existing WSL-based Gateway to a local Windows Gateway in an MXC Session.
• Which Gateway configuration, credentials, identity, sessions, and user data are preserved or moved.
• How the Companion App detects an existing installation and offers migration.
• How to verify that the new Gateway and Windows Node are connected and healthy.
• How to recover, retry, or roll back safely if migration fails.
• How to continue connecting to an existing remote Gateway instead of migrating.
• Known limitations and where to find diagnostics or support.

Acceptance criteria

• The README contains a discoverable, step-by-step migration guide.
• The guide clearly distinguishes the Gateway Session from per-tool process containment.
• Prerequisites and unsupported configurations are documented before any destructive step.
• Data-preservation and rollback behavior are stated explicitly.
• Commands, UI labels, and paths match the shipping Companion App experience.
• The guide links to the detailed architecture, setup, testing, and troubleshooting documentation where appropriate.

Start with the repository README and compare the documented flow against the shipping Companion App experience, using the detailed architecture, setup, testing, and troubleshooting documentation mentioned in the issue. Done means the README has a discoverable step-by-step migration guide covering prerequisites, preservation, verification, recovery, rollback, remote Gateway use, limitations, and diagnostics without unsupported details.