refactor: rename alknet-secret to alknet-vault

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.

crates/alknet-vault/tests/service_tests.rs

@@ -0,0 +1,98 @@
+//! Integration tests for the VaultService lifecycle.
+//!
+//! These tests verify the unlock/lock lifecycle, error conditions,
+//! and that the vault correctly manages state transitions.
+
+use alknet_vault::derivation::PATHS;
+use alknet_vault::service::{VaultServiceError, VaultServiceHandle};
+
+#[test]
+fn test_full_lifecycle() {
+    let service = VaultServiceHandle::new();
+
+    // Starts locked
+    assert!(!service.is_unlocked());
+
+    // Can't derive while locked
+    let result = service.derive_ed25519(PATHS::IDENTITY);
+    assert!(matches!(result, Err(VaultServiceError::VaultLocked)));
+
+    // Unlock
+    let phrase = service.unlock_new(24).unwrap();
+    assert!(service.is_unlocked());
+    assert!(!phrase.is_empty());
+
+    // Can derive while unlocked
+    let key = service.derive_ed25519(PATHS::IDENTITY).unwrap();
+    assert!(!key.private_key.is_empty());
+
+    // Lock
+    service.lock();
+    assert!(!service.is_unlocked());
+
+    // Can't derive again
+    let result = service.derive_ed25519(PATHS::IDENTITY);
+    assert!(matches!(result, Err(VaultServiceError::VaultLocked)));
+}
+
+#[test]
+fn test_unlock_with_known_phrase() {
+    let service = VaultServiceHandle::new();
+
+    // Generate a phrase
+    let phrase = service.unlock_new(24).unwrap();
+    service.lock();
+
+    // Re-unlock with the same phrase
+    service.unlock(&phrase, None).unwrap();
+    assert!(service.is_unlocked());
+
+    // Different passphrase produces different seed
+    // (tested by deriving keys with different passphrases)
+}
+
+#[test]
+fn test_double_unlock_fails() {
+    let service = VaultServiceHandle::new();
+    service.unlock_new(24).unwrap();
+
+    let result = service.unlock_new(12);
+    assert!(matches!(result, Err(VaultServiceError::AlreadyUnlocked)));
+}
+
+#[test]
+fn test_lock_when_already_locked_is_noop() {
+    let service = VaultServiceHandle::new();
+    assert!(!service.is_unlocked());
+
+    // Lock on already-locked service is a no-op
+    service.lock();
+    assert!(!service.is_unlocked());
+}
+
+#[test]
+fn test_encrypt_decrypt_lifecycle() {
+    let service = VaultServiceHandle::new();
+    service.unlock_new(24).unwrap();
+
+    let plaintext = "my-api-key-12345";
+    let encrypted = service.encrypt(plaintext, 1).unwrap();
+    let decrypted = service.decrypt(&encrypted).unwrap();
+    assert_eq!(decrypted, plaintext);
+
+    // After lock, can't decrypt
+    service.lock();
+    let result = service.decrypt(&encrypted);
+    assert!(matches!(result, Err(VaultServiceError::VaultLocked)));
+}
+
+#[test]
+fn test_multiple_derive_paths_succeed() {
+    let service = VaultServiceHandle::new();
+    service.unlock_new(24).unwrap();
+
+    // All standard paths should work
+    let _identity = service.derive_ed25519(PATHS::IDENTITY).unwrap();
+    let _ssh = service.derive_ed25519(PATHS::SSH_HOST).unwrap();
+    let _enc = service.derive_encryption_key(PATHS::ENCRYPTION).unwrap();
+}