[Realtime] addons/websocket package + FeatureRealtime + add/config wiring

[cssbruno]

Package the realtime WebSocket addon and feature wiring.

Scope:

• Add an optional addons/websocket package or equivalent addon boundary.
• Add FeatureRealtime and config round-trip support.
• Wire gowdk add support if the addon command is available.
• Keep SSE as the dependency-free core default.

Acceptance criteria:

• WebSocket transport remains dependency-isolated from the root module.
• Config loading recognizes the addon.
• Docs explain when to use SSE versus WebSocket.