feat(vault-tui): implement KV client and service for managing secrets

- Added internal/vault/client.go for creating a Vault client with configuration settings.
- Introduced internal/vault/errors.go to classify Vault API errors for better UI handling.
- Created internal/vault/kv.go to manage KV secrets, including listing, reading, writing, and deleting operations.
- Implemented internal/vault/mounts.go to list and describe secret engine mounts.
- Developed internal/vault/service.go to provide a unified entry point for Vault operations.
- Added internal/vault/kv_test.go for comprehensive testing of KV operations.
- Introduced internal/ui/toast.go for transient notifications in the UI.
- Added renovate.json for dependency management and updates.

internal/token/store.go

@@ -0,0 +1,31 @@
+// Package token owns everything about where a Vault token lives between
+// runs: resolving one from flag/env/config/disk, validating it, keeping it
+// renewed, and writing it back out — including in a form the real Vault CLI
+// can read (see store_vaultcli.go).
+package token
+
+import "context"
+
+// Store persists a single token value. Implementations must not log the
+// token value anywhere.
+type Store interface {
+	// Kind identifies the implementation for display: "vault-cli", "profile",
+	// or "none".
+	Kind() string
+	// Location is a display string: the token file path, or the external
+	// helper binary path.
+	Location() string
+	Get(ctx context.Context) (string, error)
+	Store(ctx context.Context, token string) error
+	Erase(ctx context.Context) error
+}
+
+// noneStore never persists anything; used when token.storage: none.
+type noneStore struct{}
+
+func NewNoneStore() Store                             { return noneStore{} }
+func (noneStore) Kind() string                        { return "none" }
+func (noneStore) Location() string                    { return "" }
+func (noneStore) Get(context.Context) (string, error) { return "", nil }
+func (noneStore) Store(context.Context, string) error { return nil }
+func (noneStore) Erase(context.Context) error         { return nil }