secret list gives each secret's own creation and last-update times (closes #142)

secret list and secret list --json gave the time of the listing as every
secret's creation and last-update time, because Secret.LoadMetadata set
both to the current time. It now reads file modification times, so
listing still needs no passphrase: the last update is when the secret's
current file was written, and the creation is when its oldest version's
value.age was written. A move to another vault gives the copied files
the times of the originals.

GetSecretObject no longer loads the times, so secret get keeps its error
for a missing current file; secret list calls LoadMetadata itself.

Judgement call: the creation time comes from value.age, not from the
oldest version's directory, whose time changes when the next add
rewrites that version's metadata.age.

Model: opus-5-5

README.md

@@ -150,6 +150,13 @@ Retrieves and outputs a secret value to stdout.
 
 Lists all secrets in the current vault. Optional filter for substring matching.
 
+The `LAST UPDATED` column, and `updated_at` in the `--json` output, give the
+time the secret's `current` file was last written: every command that adds a
+version replaces it, and so does `secret version promote`. `created_at` gives
+the time the `value.age` of the secret's oldest version was written. Both are
+the files' modification times, so listing asks for no passphrase. Moving a
+secret to another vault keeps both times.
+
 #### `secret remove <secret-name> [--force]` / `secret rm` ⚠️ 🛑
 
 **DANGER**: Permanently removes a secret and ALL its versions. It first asks for
@@ -181,7 +188,9 @@ period.
 #### `secret version promote <secret-name> <version>`
 
 Promotes a specific version to current by rewriting the secret's `current` file
-to name it. Does not modify any timestamps, allowing for rollback scenarios.
+to name it. The versions' own times are left unchanged, allowing for rollback
+scenarios; `secret list` then gives the time of the promote as the secret's last
+update.
 
 #### `secret version remove <secret-name> <version> [--force]` / `secret version rm` ⚠️ 🛑