Update golangci-lint to v2.12.2 with canonical config (closes #30)

- Replace .golangci.yml with the canonical strict config (all linters
  enabled except the standard disable list; lll 88, funlen 80/50,
  cyclop 15, dupl 100; test files now linted)
- Pin the Dockerfile lint stage to golangci/golangci-lint:v2.12.2 by
  tag and digest (Debian-based)
- Fix all ~1550 findings surfaced by the new config: line wrapping,
  wsl_v5/nlreturn blank lines, noinlineerr splits, err113 sentinel
  errors, perfsprint/modernize rewrites, goconst constants, thelper,
  testifylint, noctx CommandContext, testpackage conversions,
  t.Parallel() where safe, and complexity/dupl helper extraction
- Record the change and follow-up items in TODO.md

User-visible strings
--------------------

No user-visible string changes remain. Every error message this branch
composes is byte-identical to the one main composes.

The err113 sentinels are shaped so that fmt.Errorf reassembles the
original text around them: a sentinel carries the fixed words of the
message and the caller supplies the interpolated value in the position
it has always occupied. Where the value sits in the middle of the
sentence the sentinel therefore holds only a fragment (for example
vault.ErrVaultNotFound is "does not exist", composed by its caller as
"vault <name> does not exist"); each such sentinel documents the
message it participates in.

Verified mechanically rather than by inspection: every fmt.Errorf and
errors.New call site in both trees was parsed, the Error() text of any
sentinel passed to %w substituted in, and the resulting sets of
composed message templates compared. All 350 templates main produces
are still produced, character for character; the set of messages lost
or altered is empty.

unlocker list
-------------

findUnlockerIDByMetadata now returns (string, error) instead of
signalling failure with an empty ID. An unreadable unlockers.d is no
longer indistinguishable from "no matching entry", so UnlockersList
skips the entry with a warning naming the directory, as it did before
the scan was extracted into a helper, rather than emitting a row under
a synthesized fallback ID that no unlocker remove or unlocker select
can match and that suppresses the current-unlocker marker. The
duplicate-check and shell-completion callers skip on the same
condition, matching their pre-extraction behavior. Covered by tests in
internal/cli/unlockers_list_test.go.

internal/cli/generate.go

@@ -2,6 +2,7 @@ package cli
 
 import (
 	"crypto/rand"
+	"errors"
 	"fmt"
 	"math/big"
 	"os"
@@ -17,6 +18,16 @@ const (
 	mnemonicEntropyBits = 128
 )
 
+// Sentinel errors for secret generation
+var (
+	errLengthTooSmall           = errors.New("length must be at least 1")
+	errLengthNotPositive        = errors.New("length must be positive")
+	errMnemonicTypeNotSupported = errors.New(
+		"mnemonic type not supported for secret generation, " +
+			"use 'secret generate mnemonic' instead")
+	errUnsupportedSecretType = errors.New("unsupported type")
+)
+
 func newGenerateCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "generate",
@@ -52,8 +63,9 @@ func newGenerateSecretCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "secret <name>",
 		Short: "Generate a random secret and store it in the vault",
-		Long:  `Generate a cryptographically secure random secret and store it in the current vault under the given name.`,
-		Args:  cobra.ExactArgs(1),
+		Long: `Generate a cryptographically secure random secret and ` +
+			`store it in the current vault under the given name.`,
+		Args: cobra.ExactArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			length, _ := cmd.Flags().GetInt("length")
 			secretType, _ := cmd.Flags().GetString("type")
@@ -68,8 +80,10 @@ func newGenerateSecretCmd() *cobra.Command {
 		},
 	}
 
-	cmd.Flags().IntP("length", "l", defaultSecretLength, "Length of the generated secret (default 16)")
-	cmd.Flags().StringP("type", "t", "base58", "Type of secret to generate (base58, alnum)")
+	cmd.Flags().IntP("length", "l", defaultSecretLength,
+		"Length of the generated secret (default 16)")
+	cmd.Flags().StringP("type", "t", "base58",
+		"Type of secret to generate (base58, alnum)")
 	cmd.Flags().BoolP("force", "f", false, "Overwrite existing secret")
 
 	return cmd
@@ -98,7 +112,8 @@ func (cli *Instance) GenerateMnemonic(cmd *cobra.Command) error {
 	fmt.Fprintln(os.Stderr, "   • Write it down on paper and store it safely")
 	fmt.Fprintln(os.Stderr, "   • Do not store it digitally or share it with anyone")
 	fmt.Fprintln(os.Stderr, "   • You will need this phrase to recover your secrets")
-	fmt.Fprintln(os.Stderr, "   • If you lose this phrase, your secrets cannot be recovered")
+	fmt.Fprintln(os.Stderr,
+		"   • If you lose this phrase, your secrets cannot be recovered")
 	fmt.Fprintln(os.Stderr, "")
 	fmt.Fprintln(os.Stderr, "Use this mnemonic with:")
 	fmt.Fprintln(os.Stderr, "   secret init    (to initialize a new secret manager)")
@@ -116,11 +131,13 @@ func (cli *Instance) GenerateSecret(
 	force bool,
 ) error {
 	if length < 1 {
-		return fmt.Errorf("length must be at least 1")
+		return errLengthTooSmall
 	}
 
-	var secretValue string
-	var err error
+	var (
+		secretValue string
+		err         error
+	)
 
 	switch secretType {
 	case "base58":
@@ -128,9 +145,10 @@ func (cli *Instance) GenerateSecret(
 	case "alnum":
 		secretValue, err = generateRandomAlnum(length)
 	case "mnemonic":
-		return fmt.Errorf("mnemonic type not supported for secret generation, use 'secret generate mnemonic' instead")
+		return errMnemonicTypeNotSupported
 	default:
-		return fmt.Errorf("unsupported type: %s (supported: base58, alnum)", secretType)
+		return fmt.Errorf("%w: %s (supported: base58, alnum)",
+			errUnsupportedSecretType, secretType)
 	}
 
 	if err != nil {
@@ -147,11 +165,13 @@ func (cli *Instance) GenerateSecret(
 	secretBuffer := memguard.NewBufferFromBytes([]byte(secretValue))
 	defer secretBuffer.Destroy()
 
-	if err := vlt.AddSecret(secretName, secretBuffer, force); err != nil {
+	err = vlt.AddSecret(secretName, secretBuffer, force)
+	if err != nil {
 		return err
 	}
 
-	cmd.Printf("Generated and stored %d-character %s secret: %s\n", length, secretType, secretName)
+	cmd.Printf("Generated and stored %d-character %s secret: %s\n",
+		length, secretType, secretName)
 
 	return nil
 }
@@ -170,10 +190,11 @@ func generateRandomAlnum(length int) (string, error) {
 	return generateRandomString(length, alnumChars)
 }
 
-// generateRandomString generates a random string of the specified length using the given character set
+// generateRandomString generates a random string of the specified length
+// using the given character set
 func generateRandomString(length int, charset string) (string, error) {
 	if length <= 0 {
-		return "", fmt.Errorf("length must be positive")
+		return "", errLengthNotPositive
 	}
 
 	result := make([]byte, length)
@@ -184,6 +205,7 @@ func generateRandomString(length int, charset string) (string, error) {
 		if err != nil {
 			return "", fmt.Errorf("failed to generate random number: %w", err)
 		}
+
 		result[i] = charset[randomIndex.Int64()]
 	}