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

README.md

@@ -113,7 +113,9 @@ automatically switch to another vault if removing the current one.
 Adds a secret to the current vault. Reads the secret value from stdin.
 - `--force, -f`: Overwrite existing secret
 
-**Secret Name Format:** `[a-z0-9\.\-\_\/]+`
+**Secret Name Format:** only ASCII letters, digits, `.`, `-`, `_` and `/`
+are allowed, and a name must not be empty, start with `.` or `/`, end with
+`/`, contain `//`, or have `..` as a path segment.
 - Forward slashes (`/`) are converted to percent signs (`%`) for storage
 - Examples: `database/password`, `api.key`, `ssh_private_key`