feat(cli/device): current_device_seed + current_device_pubkey helpers

Read the active device's ed25519 seed/pubkey from
devices/<name>/signing.{key,pub}. Adds ssh-key (0.6) as a CLI dep
(already at 0.6.7 in the workspace lock via relicario-core) and
ed25519-dalek as a dev-dep for the round-trip test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2166,6 +2166,7 @@ dependencies = [
  "clap_complete",
  "data-encoding",
  "dirs",
+ "ed25519-dalek",
  "hex",
  "image",
  "predicates",
@@ -2177,6 +2178,7 @@ dependencies = [
  "rqrr",
  "serde",
  "serde_json",
+ "ssh-key",
  "tar",
  "tempfile",
  "url",