Reject invalid secret names before any command builds a path (closes #33)

`secret rm ..` deleted the whole vault; `secret rm .` and `secret rm ""`
deleted every secret. rm, mv, the version commands, encrypt and decrypt
built paths from the name unchecked; import checked it only after
reading the source file.

Each now calls vault.ValidateSecretName, which wraps the existing rule,
on the name as given, before building any path; MoveSecret checks both
names before switching the current vault. Its error and README.md state
the rule. The test-only copy of the rule in internal/secret is removed.

The regression test runs each rejected command on a copy of two
in-memory vaults and requires the exact error and an unchanged state
directory.

Model: opus-5-5

internal/secret/secret_test.go

@@ -310,64 +310,6 @@ func TestPerSecretKeyFunctionality(t *testing.T) {
 	})
 }
 
-// For testing purposes only
-func isValidSecretName(name string) bool {
-	if name == "" {
-		return false
-	}
-	// Valid characters for secret names: letters, numbers, dash, dot, underscore, slash
-	for _, char := range name {
-		if (char < 'a' || char > 'z') && // lowercase letters
-			(char < 'A' || char > 'Z') && // uppercase letters
-			(char < '0' || char > '9') && // numbers
-			char != '-' && // dash
-			char != '.' && // dot
-			char != '_' && // underscore
-			char != '/' { // slash
-			return false
-		}
-	}
-
-	return true
-}
-
-func TestSecretNameValidation(t *testing.T) {
-	t.Parallel()
-
-	tests := []struct {
-		name  string
-		valid bool
-	}{
-		{"valid-name", true},
-		{"valid.name", true},
-		{"valid_name", true},
-		{"valid/path/name", true},
-		{"123valid", true},
-		{"", false},
-		{"Valid-Upper-Name", true}, // uppercase allowed
-		{"2025-11-21-ber1app1-vaultik-test-bucket-AKI", true}, // real-world uppercase key ID
-		{"MixedCase/Path/Name", true},                         // mixed case with path
-		{"invalid name", false},                               // space not allowed
-		{"invalid@name", false},                               // @ not allowed
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			t.Parallel()
-
-			result := isValidSecretName(test.name)
-			if result != test.valid {
-				t.Errorf(
-					"isValidSecretName(%q) = %v, want %v",
-					test.name,
-					result,
-					test.valid,
-				)
-			}
-		})
-	}
-}
-
 func TestSecretGetValueWithEnvMnemonicUsesVaultDerivationIndex(t *testing.T) {
 	// This test demonstrates the bug where GetValue uses hardcoded index 0
 	// instead of the vault's actual derivation index when using environment mnemonic