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/secret/metadata.go

@@ -8,11 +8,11 @@ import (
 type VaultMetadata struct {
 	CreatedAt       time.Time `json:"createdAt"`
 	Description     string    `json:"description,omitempty"`
-	DerivationIndex uint32    `json:"derivation_index"`
+	DerivationIndex uint32    `json:"derivationIndex"`
 	// Double SHA256 hash of the actual long-term public key
-	PublicKeyHash string `json:"public_key_hash,omitempty"`
+	PublicKeyHash string `json:"publicKeyHash,omitempty"`
 	// Double SHA256 hash of index-0 key (for grouping vaults from same mnemonic)
-	MnemonicFamilyHash string `json:"mnemonic_family_hash,omitempty"`
+	MnemonicFamilyHash string `json:"mnemonicFamilyHash,omitempty"`
 }
 
 // UnlockerMetadata contains information about an unlocker