Update auth references from JWT to session tokens

- Remove PreviousTokenGraceUntil from RotateApiTokenResponse
- Update comments and CLI flag descriptions
- Remove grace period test from integration test
- Update README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

auth.go

@@ -49,7 +49,7 @@ func Register(baseURL, email, password string, opts ...RegisterOption) (*AuthReg
 	return &result, nil
 }
 
-// Login authenticates a user and returns a JWT token.
+// Login authenticates a user and returns a session token.
 func Login(baseURL, email, password string) (*AuthLoginResponse, error) {
 	if baseURL == "" {
 		baseURL = defaultBaseURL