feat(agent-setup): add skill to derive and export GH_CONFIG_DIR from AGENT_HOME

Provides a session dotfile ($AGENT_HOME/.env) that subsequent skills
can source to inherit GH_CONFIG_DIR.

agent-setup/scripts/setup.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+die() { echo "ERROR: $*" >&2; exit 1; }
+
+[[ -z "${AGENT_HOME:-}" ]] && die "AGENT_HOME is not set"
+
+# Derive GH_CONFIG_DIR — gh stores config at ~/.config/gh by default,
+# so we mirror that structure under AGENT_HOME
+export GH_CONFIG_DIR="$AGENT_HOME/.github"
+
+# Write to a session dotfile so child processes inherit the variables
+mkdir -p "$AGENT_HOME"
+cat > "$AGENT_HOME/.env" <<EOF
+export GH_CONFIG_DIR="$GH_CONFIG_DIR"
+EOF
+
+echo "GH_CONFIG_DIR set to $GH_CONFIG_DIR"