feat(mesh): autonomous windows provisioning and lifecycle management ...

- Implemented secure self-purge mechanism for agent nodes (purge.py).
- Refactored bootstrap_windows.ps1 for headless/non-interactive installation.
- Added POST /nodes/purge endpoint to Hub for token-based self-deregistration.
- Enhanced Nodes UI with 'Purge & Deregister' option and platform-aware command dispatch.
- Modularized backend services (Mesh, NodeRegistry, Session, Tool, RAG) to centralize business logic.
- Stabilized Windows PTY serialization and terminal output dispatch.
- Added integration tests for node purge flows.