chore: silence bascule-shell config dead-code warnings

The TOML schema for ~/.config/bascule/shell.toml carries
`servers = [{alias, hostname, port}]` entries that
bascule-shell deserializes but doesn't read at runtime. The
shell-side server chooser uses ssh host aliases (dev.gsh /
stg.gsh / prod.gsh) instead.

Marking the fields `#[allow(dead_code)]` with a comment
preserves the TOML wire format (so users with existing config
files don't get a parse error) without leaving the compiler
warning.

Verification:
  $ cargo build --workspace
    | grep -c "warning:"
  0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Claude Code <claude@guildhouse.dev>

crates/bascule-shell/src/config.rs

@@ -6,13 +6,21 @@ pub struct ShellConfig {
     pub inner_shell: String,
     #[serde(default = "default_true")]
     pub show_banner: bool,
+    /// User-configured Bascule server endpoints. The bascule-shell
+    /// binary deserializes them from `~/.config/bascule/shell.toml`
+    /// but never reads them at runtime today; the shell-side server
+    /// chooser uses `dev.gsh` / `stg.gsh` / `prod.gsh` SSH host
+    /// aliases instead. Retained so the TOML schema doesn't break
+    /// for users who already have entries in their config.
     #[serde(default)]
+    #[allow(dead_code)]
     pub servers: Vec<BasculeServer>,
     #[serde(default = "default_pcr_indices")]
     pub pcr_indices: Vec<u32>,
 }
 
 #[derive(Debug, Deserialize, Clone)]
+#[allow(dead_code)] // TOML wire-format fields; see ShellConfig.servers comment.
 pub struct BasculeServer {
     pub alias: String,
     pub hostname: String,