Rename the crate from alknet-secret to alknet-vault to better reflect its
purpose as a local key vault (seed management, key derivation, encryption)
rather than a network service.
Symbol renames:
- SecretService → VaultService
- SecretServiceHandle → VaultServiceHandle
- SecretServiceActor → VaultServiceActor
- SecretServiceError → VaultServiceError
- SecretProtocol → VaultProtocol
- SecretMessage → VaultMessage
- ServiceLocked → VaultLocked
- alknet_secret → alknet_vault (crate name)
Update ADR-008 with vault access pattern: the vault is a capability source,
not a service endpoint. The CLI injects derived/decrypted material into
operation contexts — handlers never hold vault references.
