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_vaultcli.go

@@ -0,0 +1,51 @@
+package token
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/hashicorp/vault/api/cliconfig"
+	"github.com/hashicorp/vault/api/tokenhelper"
+)
+
+// vaultCLIStore delegates to the exact packages the Vault CLI itself uses,
+// so ~/.vault-token stays byte-compatible and any token_helper configured in
+// ~/.vault (HCL file, overridable via VAULT_CONFIG_PATH) is honoured. Both
+// api/cliconfig and api/tokenhelper ship inside the api module we already
+// depend on, so this costs zero additional dependencies.
+type vaultCLIStore struct {
+	h tokenhelper.TokenHelper
+}
+
+// NewVaultCLIStore resolves the configured token helper (falling back to
+// the internal ~/.vault-token helper when none is configured).
+func NewVaultCLIStore() (Store, error) {
+	h, err := cliconfig.DefaultTokenHelper()
+	if err != nil {
+		return nil, fmt.Errorf("resolving vault token helper: %w", err)
+	}
+	return vaultCLIStore{h: h}, nil
+}
+
+func (s vaultCLIStore) Kind() string { return "vault-cli" }
+
+// Location calls Get first: InternalTokenHelper.Path() is only populated
+// after its first Get/Store/Erase call (it lazily resolves the home
+// directory internally), so an unconditional Path() call before any other
+// operation would return "".
+func (s vaultCLIStore) Location() string {
+	_, _ = s.h.Get()
+	return s.h.Path()
+}
+
+func (s vaultCLIStore) Get(context.Context) (string, error) {
+	return s.h.Get()
+}
+
+func (s vaultCLIStore) Store(_ context.Context, tok string) error {
+	return s.h.Store(tok)
+}
+
+func (s vaultCLIStore) Erase(context.Context) error {
+	return s.h.Erase()
+}