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

`secret rm ..` resolved to the vault directory and 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
without checking it; import checked it only after reading the source
file.

vault.ValidateSecretName wraps the existing name rule and its error
states the rule. Each of those commands calls it on the name as given,
before building any path; a move checks both names before switching
the current vault. AddSecret, GetSecretVersion and GetSecretObject use
it too.

The regression test copies two in-memory vaults for each rejected
command and requires the exact error and an unchanged state directory.

Model: opus-5-5

internal/vault/errors.go

@@ -29,10 +29,11 @@ var (
 	// ErrNilValueBuffer indicates a nil value buffer was supplied.
 	ErrNilValueBuffer = errors.New("value buffer is nil")
 
-	// ErrInvalidSecretName indicates a secret name that does not match
-	// the allowed pattern [a-z0-9.\-_/]+. Composed as
-	// "invalid secret name '<name>': must match pattern [a-z0-9.\-_/]+",
-	// or as "invalid secret name: <name>" by GetSecretObject.
+	// ErrInvalidSecretName indicates a secret name that breaks the naming
+	// rule: only letters, digits, '.', '-', '_' and '/'; not empty; no
+	// leading '.' or '/', no trailing '/', no '//', no '..' path segment.
+	// Composed by ValidateSecretName as
+	// "invalid secret name '<name>': <the rule>".
 	ErrInvalidSecretName = errors.New("invalid secret name")
 
 	// ErrSecretExists indicates the secret already exists and --force