feat(code): friendly errors, batch history, persistent command history

- Agent errors sanitized: raw hyper/h2/gRPC dumps replaced with
  human-readable messages ("sol disconnected", "connection lost", etc.)
- Batch history loading: single viewport rebuild instead of per-entry
- Persistent command history: saved to .sunbeam/history, loaded on start
- Default model: mistral-medium-latest (personality adherence)

sunbeam/src/code/mod.rs

@@ -82,7 +82,7 @@ async fn cmd_code_inner(cmd: Option<CodeCommand>) -> anyhow::Result<()> {
 
             let model = model
                 .or(cfg.model_name.clone())
-                .unwrap_or_else(|| "devstral-small-latest".into());
+                .unwrap_or_else(|| "mistral-medium-latest".into());
 
             // Connect to Sol
             let mut session = client::connect(&endpoint, &project, &cfg, &model).await?;
@@ -104,7 +104,7 @@ async fn cmd_code_inner(cmd: Option<CodeCommand>) -> anyhow::Result<()> {
 
             // Spawn agent on background task
             let project_path = project.path.clone();
-            let agent = agent::spawn(session);
+            let agent = agent::spawn(session, endpoint.clone());
 
             // TUI event loop — never blocks on network I/O
             use crossterm::event::{self, Event, KeyCode, KeyModifiers, MouseEventKind};
@@ -160,6 +160,12 @@ async fn cmd_code_inner(cmd: Option<CodeCommand>) -> anyhow::Result<()> {
                             app.sol_status.clear();
                             app.push_log(tui::LogEntry::Error(message));
                         }
+                        agent::AgentEvent::Health { connected } => {
+                            if app.sol_connected != connected {
+                                app.sol_connected = connected;
+                                app.needs_redraw = true;
+                            }
+                        }
                         agent::AgentEvent::SessionEnded => {
                             break;
                         }