Function setup_agent 

pub fn setup_agent(
    config: AgentSetupConfig,
    ctx: &AuthsContext,
    keychain: Box<dyn KeyStorage + Send + Sync>,
    passphrase_provider: &dyn PassphraseProvider,
) -> Result<AgentSetupResult, SetupError>

Provisions an agent identity delegated from a parent identity.

Constructs all proposed state first, then persists only if dry_run is false.

Args:

• config: Agent setup parameters (alias, parent DID, capabilities, etc.).
• ctx: Injected infrastructure adapters (registry, clock).
• keychain: Platform keychain for key storage.
• passphrase_provider: Provides passphrases for key operations.

Usage:

let result = setup_agent(config, &ctx, keychain, &provider)?;