Terminal: show the shell's error output, so a missing shell is explained

The shell's stderr was discarded, so a shell that could not start looked like an
immediate disconnect. It now goes to the terminal like other output, followed by
the exit code. An end-to-end test covers a machine with no shell. Builds against
Bamboo v0.1.10, which says why when there is no shell to run.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

go.mod

@@ -3,7 +3,7 @@ module git.veillax.com/Veillax/hammerhead
 go 1.27.3
 
 require (
-	git.veillax.com/Veillax/bamboo v0.1.9
+	git.veillax.com/Veillax/bamboo v0.1.10
 	git.veillax.com/Veillax/blacktip v0.1.10
 	git.veillax.com/Veillax/reef v0.1.12
 	github.com/emersion/go-msgauth v0.7.0