refactor: remove legacy chat path, fix corrupted conversation recovery

- Delete CodeSession::chat() — the legacy inline tool loop that
  duplicated the orchestrator's conversation + tool dispatch logic
- Delete wait_for_tool_result() — only used by the legacy path
- Make orchestrator mandatory in run_session (no more if/else fallback)
- Unify conversation creation into create_fresh_conversation()
- Add corrupted conversation recovery to create_or_append_conversation:
  detects "function calls and responses" errors from Mistral (caused by
  disconnecting mid-tool-call) and auto-creates a fresh conversation
- Add tracing-appender for optional rotating log file (SOL_LOG_FILE env)
- Add Procfile.dev for overmind process management

src/main.rs

@@ -44,13 +44,39 @@ use tools::ToolRegistry;
 
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
-    // Initialize tracing
-    tracing_subscriber::fmt()
-        .with_env_filter(
-            tracing_subscriber::EnvFilter::try_from_default_env()
-                .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("sol=info")),
-        )
-        .init();
+    // Initialize tracing — optionally write to a rotating log file
+    let env_filter = tracing_subscriber::EnvFilter::try_from_default_env()
+        .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("sol=info"));
+
+    let _log_guard = if let Ok(log_path) = std::env::var("SOL_LOG_FILE") {
+        let log_dir = std::path::Path::new(&log_path)
+            .parent()
+            .unwrap_or(std::path::Path::new("."));
+        let log_name = std::path::Path::new(&log_path)
+            .file_name()
+            .and_then(|n| n.to_str())
+            .unwrap_or("sol.log");
+
+        let file_appender = tracing_appender::rolling::Builder::new()
+            .max_log_files(3)
+            .rotation(tracing_appender::rolling::Rotation::NEVER)
+            .filename_prefix(log_name)
+            .build(log_dir)
+            .expect("Failed to create log file appender");
+
+        let (non_blocking, guard) = tracing_appender::non_blocking(file_appender);
+        tracing_subscriber::fmt()
+            .with_env_filter(env_filter)
+            .with_writer(non_blocking)
+            .with_ansi(false)
+            .init();
+        Some(guard)
+    } else {
+        tracing_subscriber::fmt()
+            .with_env_filter(env_filter)
+            .init();
+        None
+    };
 
     // Load config
     let config_path =