Commands fail when currentvault names no vault (closes #156)

An empty currentvault, or one naming a vault that does not exist, made
`secret list` report an empty vault with exit 0 and `secret add` fail on a
missing directory. ResolveVaultSymlink now refuses an empty or invalid
name, or one with no directory under vaults.d, with an error that wraps
ErrVaultNotFound, names the file and says to run
`secret vault select <name>`. Every command that uses the current vault
gets it; `secret vault list` and `secret info` still run.

newSizeTestVault wrote a full path into currentvault, which resolved only
because the last path element was taken as the name; it now relies on
CreateVault selecting the vault.

Model: opus-5-5

README.md

@@ -117,7 +117,8 @@ are allowed, and a name must not be empty, `.` or `..`.
 Switches to the specified vault for subsequent operations. When `currentvault`
 is empty or names a vault that does not exist, every command that uses the
 current vault fails with an error that names the file and says to run
-`secret vault select <name>`; `secret vault list` still lists the vaults.
+`secret vault select <name>`. `secret vault list` still lists the vaults, and
+`secret info` still runs; both show no vault as current.
 
 #### `secret vault remove <name> [--force]` / `secret vault rm` ⚠️ 🛑
 

internal/cli/current_vault_test.go

@@ -17,15 +17,16 @@ import (
 
 // TestListFailsWhenCurrentVaultDoesNotExist is a regression test for
 // https://git.eeqj.de/sneak/secret/issues/156, where a currentvault file
-// that was empty, or named a vault that does not exist, made `secret list`
-// report an empty vault. Listing must fail with an error that names the
-// file and says to run `secret vault select`, `secret vault list` must
-// still list the vaults, and selecting a vault must make listing work
-// again.
+// that was empty, held an invalid vault name, or named a vault that does
+// not exist, made `secret list` report an empty vault. Listing must fail
+// with an error that names the file and says to run `secret vault select`,
+// `secret vault list` must still list the vaults, and selecting a vault
+// must make listing work again. The name "." would make vaults.d itself the
+// vault, and "../vaults.d/work" would reach the vault work by a path.
 func TestListFailsWhenCurrentVaultDoesNotExist(t *testing.T) {
 	t.Parallel()
 
-	for _, name := range []string{"", "gone"} {
+	for _, name := range []string{"", ".", "../vaults.d/work", "gone"} {
 		t.Run(strconv.Quote(name), func(t *testing.T) {
 			t.Parallel()