wip: in-progress changes (grid, config, wizard, settings, pty) before session-persistence

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

crates/spacesh-proto/src/message.rs

@@ -131,6 +131,8 @@ pub enum Cmd {
         surface_id: Option<SurfaceId>,
     },
     Health,
+    /// Which of the given CLI candidates are actually installed on the spawn PATH.
+    WhichAgents { candidates: Vec<String> },
     Status,
     Shutdown,
     GetConfig,