feat: upgrade default model to glm-5.1

[rodrigorodrigo]

Summary

• Change DefaultModel constant from glm-5 to glm-5.1
• Update README documentation to reflect glm-5.1 as new default
• Add glm-5.1 to available models list

Test plan

• Verify DefaultModel constant in internal/token/token.go is set to glm-5.1
• Verify README references updated to reflect glm-5.1 as default
• Verify glm-5 remains in available models list
• Build and run glm to confirm default model is glm-5.1