fix(portability): make test green on macOS, GNU-only idioms replaced

The suite and seven libs assumed a GNU userland: `cmd | grep -q` under
pipefail (grep exits at the first match, the producer takes SIGPIPE,
rc 141 → 15 false "merged into" FAILs in gitflow-test), `sed -i` with no
suffix, `wc -l` padding compared as a string, `stat -c`, `touch -d`,
`realpath -m` (gstack-links refusal never fired), bare `timeout` off the
sanitized PATH (design gate READY BUT UNVERIFIED), `grep -oP` (update-all
emptied the plugin list). Thirteen suites were red on this machine.

Portable forms on the native userland of both OS: producer captured out
of the pipeline, `sed -i.bak` in tests and a temp-sibling `sed_profile`
on the user dotfile, `tr -d ' '`, python3 perms, `touch -t`, a
`realpath -m` emulation that refuses `..`, perl `alarm` for the 15 s
bound, `sed -n` token extraction. Regression tests: gstack-links T4b,
doctrine-citers Alphabet/Alpha flip, profile-set-managed T18 (failing CLI
no longer aborts `set`), new portability-census suite over the tracked
shell files. Linux run deferred (see TODO).

lib/toggle-external.sh

@@ -139,7 +139,7 @@ pack_hints() {
     21st)
       if ! command -v 21st >/dev/null 2>&1; then
         warn "the \`21st\` CLI is not on PATH — install it: npm i -g @21st-dev/cli"
-      elif ! 21st whoami 2>/dev/null | grep -q '^Logged in as '; then
+      elif ! grep -q '^Logged in as ' <<<"$(21st whoami 2>/dev/null)"; then
         warn "not signed in to 21st — component retrieval and 21st AI need: 21st login"
       fi
       ;;