fix: review findings — env-var netrc write hygiene, network-vs-auth errors, installer collision guard, docs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

install-codex.sh

@@ -9,6 +9,10 @@ dest="${HOME}/.agents/skills"
 
 mkdir -p "$dest"
 for skill in triage setup map-repo; do
+  if [ -e "$dest/$skill" ] && [ ! -L "$dest/$skill" ]; then
+    echo "errore: $dest/$skill esiste già e non è un link simbolico — rimuovilo e rilancia" >&2
+    exit 1
+  fi
   ln -sfn "$repo_dir/skills/$skill" "$dest/$skill"
   echo "linked $dest/$skill -> $repo_dir/skills/$skill"
 done