fix(summer): pass the app binary's exit code through and print its error once

- bonfire.WithExitCode (additive) makes an error carry the exit code
  bonfire.Execute returns; a code outside 1..255 gives 1
- runAppCmd, factored out of runAppBinary, returns
  Reported(WithExitCode(err, code)) when the binary exited with a code, so
  summer prints no second error block; a binary killed by a signal keeps
  summer's block and exit 1
- README, console introduction and upgrading notes say so (CR-02)

docs/console/introduction.md

@@ -141,6 +141,8 @@ summer -v migrate --pretend
 summer admin:token:create jane --scope journal.posts --json
 ```
 
+When the binary fails, `summer` exits with the binary's exit code (a usage error stays `2`) and prints nothing more: the binary has already printed its error. A binary killed by a signal gets an error block from `summer` and exit code `1`.
+
 When `summer` receives SIGINT or SIGTERM while the binary runs, it passes the signal on and waits for the binary to shut down, so stopping `summer serve` with Ctrl+C or `kill` frees its port.
 
 `summer` does not rebuild an existing binary before it delegates. Run `summer build` after you change code, or keep `summer dev` running. Every other runtime command, such as `deferred:purge` or the commands your plugins add, is run on the binary directly.