fix(core): create session if no current and no last

If we're not starting a new session, we don't have a current session and
we don't have a last session, we have to create a new session.

Without a session, submitting prompts fails silently.

Author: cameronr

lua/opencode/core.lua

@@ -76,6 +76,9 @@ function M.open(opts)
   else
     if not state.active_session then
       state.active_session = session.get_last_workspace_session()
+      if not state.active_session then
+        state.active_session = M.create_new_session()
+      end
     else
       if not state.display_route and are_windows_closed then
         -- We're not displaying /help or something like that but we have an active session