feat(spaceshd): configurable default shell

Add a config.rs module that loads optional ~/.spacesh/config.toml and a
default_shell() resolver: SPACESH_SHELL env -> config.toml default_shell ->
login shell from the passwd DB -> $SHELL -> /bin/sh. The passwd lookup
matters under launchd where $SHELL is absent. All plain-panel spawn sites in
server.rs now use the resolver. Adds toml + libc deps.

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

crates/spaceshd/Cargo.toml

@@ -22,3 +22,5 @@ thiserror.workspace = true
 futures.workspace = true
 fs2.workspace = true
 dirs.workspace = true
+toml.workspace = true
+libc.workspace = true