Commits of agents carry a co-author trailer naming the model

A git wrapper (docker/git-coauthor -> /usr/local/lib/codecraft/bin/git) adds
--trailer "$CODECRAFT_COMMIT_TRAILER" right after the commit subcommand; the
agent shell puts it first on PATH and sets the trailer in the script itself
(works inside the sandbox, which clears the environment). Default text:
"Co-Authored-By: {model} ({provider}) via CodeCraft" (server.commit_trailer,
Settings -> General; empty = off). Git skips duplicates (amend, model wrote it).

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

docker/git-coauthor

@@ -0,0 +1,21 @@
+#!/bin/bash
+# git for the shells of CodeCraft agents: `git commit` gets the co-author trailer from
+# $CODECRAFT_COMMIT_TRAILER (model, provider, CodeCraft). Everything else goes to the real
+# git unchanged. Git itself skips the trailer when the message already ends with it.
+real=${CODECRAFT_REAL_GIT:-/usr/bin/git}
+[[ -n $CODECRAFT_COMMIT_TRAILER ]] || exec "$real" "$@"
+# The subcommand comes after git's global options (-C dir, -c key=value, --git-dir=…).
+i=1
+while (( i <= $# )); do
+  case ${!i} in
+    -C | -c | --git-dir | --work-tree | --namespace | --exec-path | --config-env) i=$((i + 2)) ;;
+    --) break ;;
+    -*) i=$((i + 1)) ;;
+    *) break ;;
+  esac
+done
+if (( i <= $# )) && [[ ${!i} == commit ]]; then
+  # Right after "commit", so it stays an option even before "-- <paths>".
+  exec "$real" "${@:1:i}" --trailer "$CODECRAFT_COMMIT_TRAILER" "${@:i+1}"
+fi
+exec "$real" "$@"