fix(bins): cygpath-normalize SCRIPT_DIR for bun imports; surface learnings-log errors (#1950)

Under Windows git-bash, pwd yields a POSIX path (/c/Users/...) that Bun on
Windows cannot resolve as an ES module specifier. gstack-learnings-log
interpolates SCRIPT_DIR into a bun -e import, so every invocation died with
"Cannot find module" — and 2>/dev/null swallowed the error, silently
dropping every AI-logged learning for Windows users.

- 3-line cygpath -m guard in gstack-learnings-log and gstack-question-log
  (which gains the same import shape in the next commit). Matches the
  duplicated IS_WINDOWS convention in setup; no shared shell lib exists.
- learnings-log adopts question-log's set +e / TMPERR capture pattern
  wholesale: validation errors now print to stderr. The old
  `if [ $? -ne 0 ]` check was dead code under set -euo pipefail — the
  script exited at the failing assignment before reaching it.
- New test/bin-windows-bun-import-paths.test.ts: static invariant (any
  bash bin interpolating $SCRIPT_DIR into a bun -e import must carry the
  guard) + behavioral end-to-end run invoked via `bash <bin>` — added to
  the windows-free-tests workflow list so the conversion is proven on the
  only platform where the bug exists.

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

bin/gstack-question-log

@@ -27,6 +27,12 @@
 # Append-only JSONL. Dedup is at read time in gstack-question-sensitivity --read-log.
 set -euo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+# Windows git-bash (#1950): pwd yields a POSIX path (/c/Users/...), which Bun
+# on Windows cannot resolve as an ES module specifier in bun -e imports.
+# cygpath -m converts to C:/Users/... which Bun accepts.
+case "$(uname -s)" in
+  MINGW*|MSYS*|CYGWIN*) command -v cygpath >/dev/null 2>&1 && SCRIPT_DIR="$(cygpath -m "$SCRIPT_DIR")" ;;
+esac
 eval "$("$SCRIPT_DIR/gstack-slug" 2>/dev/null)"
 # GSTACK_STATE_ROOT takes precedence over GSTACK_HOME (test isolation per D16).
 GSTACK_HOME="${GSTACK_STATE_ROOT:-${GSTACK_HOME:-$HOME/.gstack}}"