Refuse to create a vault that already exists (closes #74)

vault.CreateVault now checks for the vault before writing anything and
fails with "vault NAME already exists" (vault.ErrVaultExists). secret
init and secret vault create call it while holding the state directory
lock, so two creates at once cannot both pass the check. Before, either
command over an existing vault replaced its metadata, passphrase
unlocker and longterm.age, so none of its secrets could be decrypted.

Both commands now ask for the unlocker passphrase before creating the
vault, so one stopped at that prompt leaves no vault without an
unlocker behind, which they would then refuse to create again.

The lock tests set up the vault "work" instead of "default", which init
now refuses to create again.

Model: opus-5-5

TODO.md

@@ -25,6 +25,15 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-04: `secret init` refuses when the default vault exists, and
+  `secret vault create NAME` when `NAME` does, with "vault NAME already
+  exists", before writing anything. The check is in `vault.CreateVault`,
+  which both commands call while holding the state directory lock, so two
+  creates of one vault at once cannot both pass the check. Before, either
+  command replaced the vault's metadata, passphrase unlocker and
+  `longterm.age`, so none of its secrets could be decrypted any more. Both
+  commands now ask for the unlocker passphrase before creating the vault,
+  so one stopped at that prompt leaves no vault behind.
 - 2026-10-04: A PGP unlocker whose metadata has no usable GPG key ID
   no longer panics: `GetID()` warns with the unlocker's directory and
   returns `pgp-unknown`. `ListUnlockers` skips, with a warning, an
@@ -57,9 +66,9 @@ Bring the repo into policy compliance in one commit:
       has one, and to a PGP, keychain or Secure Enclave unlocker added
       on the same host and day as another of its type
       (https://git.eeqj.de/sneak/secret/issues/71);
-    - from `vault create` stopped at the passphrase prompt, a new vault
-      with no unlocker that is already the current vault; from `init`
-      stopped there, the default vault with no unlocker;
+    - from `init` or `vault create` killed after the passphrase prompt
+      but before the unlocker is written, a vault with no unlocker,
+      which `vault create` has already made the current vault;
     - from an unlocker add stopped before its metadata is written, a
       directory that `unlocker list` warns about and `unlocker rm`
       cannot remove;