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

TODO.md

@@ -25,6 +25,13 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-03: Every command that builds a path from a secret name
+  checks the name first with `vault.ValidateSecretName` and touches
+  nothing when it is invalid: `rm`, `mv` (both names, within a vault
+  and between vaults, before switching the current vault), `import`,
+  `version list`/`promote`/`rm`, `encrypt` and `decrypt`. The error
+  and `README.md` state the naming rule. Before, `secret rm ..`
+  deleted the whole vault and `secret rm .` every secret in it.
 - 2026-10-03: The keychain unlocker's age key passphrase stays in
   locked memory: it is generated into a locked buffer, and the
   keychain JSON is written and read by `KeychainData` code in
@@ -100,8 +107,7 @@ Bring the repo into policy compliance in one commit:
       to GPGEncryptFunc and EncryptWithPassphrase.
     - Race conditions: no file locking in vault/secrets.go:142-176;
       non-atomic writes can leave the vault inconsistent.
-    - Input validation: dots in secret names risk path traversal
-      (vault/secrets.go:75-99); no maximum secret size (DoS).
+    - Input validation: no maximum secret size (DoS).
     - Timing attacks: bytes.Equal passphrase compare (cli/init.go:
       209-216); non-constant-time public key compare (vault.go:95-100).
 - High priority: