fix: resolve all remaining linter issues (staticcheck, tagliatelle, lll)

- Fix staticcheck QF1011: Remove explicit type declaration for io.Writer variables
- Fix tagliatelle: Change all JSON tags from snake_case to camelCase
  - created_at → createdAt
  - keychain_item_name → keychainItemName
  - age_public_key → agePublicKey
  - age_priv_key_passphrase → agePrivKeyPassphrase
  - encrypted_longterm_key → encryptedLongtermKey
  - derivation_index → derivationIndex
  - public_key_hash → publicKeyHash
  - mnemonic_family_hash → mnemonicFamilyHash
  - gpg_key_id → gpgKeyId
- Fix lll: Break long function signature line to stay under 120 character limit

All linter issues have been resolved. The codebase now passes all linter checks.

internal/vault/management.go

@@ -197,7 +197,8 @@ func ListVaults(fs afero.Fs, stateDir string) ([]string, error) {
 }
 
 // processMnemonicForVault handles mnemonic processing for vault creation
-func processMnemonicForVault(fs afero.Fs, stateDir, vaultDir, vaultName string) (derivationIndex uint32, publicKeyHash string, familyHash string, err error) {
+func processMnemonicForVault(fs afero.Fs, stateDir, vaultDir, vaultName string) (
+	derivationIndex uint32, publicKeyHash string, familyHash string, err error) {
 	// Check if mnemonic is available in environment
 	mnemonic := os.Getenv(secret.EnvMnemonic)