chore(deps): drop unused russh-keys dependency

Zero references in src/ — ssh.rs uses russh's own russh::keys module (russh
0.61 merged key handling in-tree; the standalone russh-keys crate is a leftover
pin). cargo tree -i confirmed nothing else pulls it; gone from Cargo.lock after
removal. Build, make check, and cargo install --locked all green.

Cargo.toml

@@ -7,7 +7,6 @@ edition = "2024"
 clap = { version = "4.6", features = ["derive"] }
 tokio = { version = "1", features = ["full"] }
 russh = "0.61"
-russh-keys = "0.50.0-beta.7"
 anyhow = "1.0"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1"