feat(identity): implement X25519 keypair generation and file storage

[ethical-buddy]

Implemented the Identity layer that handles Noise-compatible X25519 keypair generation, loading, and saving.

This layer is essential to:

Generate and persist local identity (Noise keypairs)

Retrieve the keypair during handshake sessions

Store/load identities for repeated secure communication

I added a temp. test in main.go to review and run use "kp, _ := identity.GenerateKeypair()
fmt.Printf("Public Key: %x\n", kp.PublicKey)
identity.SaveKeypair(kp, "mykey.json")

loaded, _ := identity.LoadKeypair("mykey.json")
fmt.Printf("Loaded Key: %x\n", loaded.PublicKey)

" as guide.

[JumaOchi]

Looks good maybe look into default paths maybe might be helpfull to add a utility function for default keypair locations. Also Maybe some error handling i.e code currently ignores the error from GenerateKeypair(). Other than that looks solid