feat: add Platform Account authentication

[arjankowski]

Adds a new authentication type, Platform Account. A Platform Account environment authenticates with a JWT signed by the app's key pair and always acts as the single Platform Account defined in the config file.

• configure:environments:add --platform-account-auth adds a Platform Account environment from the downloaded credentials file. The flag is exclusive with --ccg-auth and --ccg-user.
• Both clients are supported. TS SDK commands use BoxPlatformAccountAuth directly. Legacy SDK commands use PlatformAccountSessionAdapter, which requests, caches, and refreshes tokens through the same auth object.
• The principal is fixed. --as-user, a default As-User (switch-user), the bulk as-user field, tokens:get --user-id, and configure:environments:update --user-id are rejected with a clear error.
• tokens:exchange is not supported, because Platform Account tokens cannot be downscoped.
• The config file is validated when the environment is added or updated and on every run, with messages that name the missing field.
• Docs: new "Platform Account Auth" section in docs/authentication.md, plus regenerated command docs.