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>

crates/relicario-cli/Cargo.toml

@@ -30,9 +30,11 @@ image = { version = "0.25", default-features = false, features = ["jpeg", "png"]
 rqrr = "0.7"
 reqwest = { version = "0.12", features = ["blocking", "json"] }
 qrcode = { version = "0.14", features = ["svg"] }
+ssh-key = { version = "0.6", features = ["ed25519", "std"] }
 
 [dev-dependencies]
 assert_cmd = "2"
 predicates = "3"
 tempfile = "3"
 serde_json = "1"
+ed25519-dalek = "2"