2.0.0: changelog and skill docs for slugs and environments

Claude-Session: https://claude.ai/code/session_01SMCvdwDmuxoaqGgvGBLk1V

skills/rudder-web-sdk/reference/auth.md

@@ -50,6 +50,19 @@ On success both tokens are saved to the client's `TokenStore`, the runtime
 starts (domains warmed, scenario engine restored, `player_login` event fired),
 and the state flips to `'signed-in'`.
 
+## Environments
+
+A project has two environments, `staging` and `prod`. The SDK key you pass as
+`RudderClientOptions.projectKey` belongs to one of them, so the environment is
+resolved at login and carried inside the access and refresh tokens; nothing in
+the client API takes an environment argument, and a player created in one
+environment is invisible in the other. Content released only to `staging` is
+empty for a `prod` key and vice versa.
+
+Tokens issued before SDK 2.0.0 carry no environment claim and are rejected with
+401. The transport's refresh then fails, clears the token store and emits
+`'signed-out'` — log the player in again.
+
 ## Auth state
 
 ```ts