chore(memory): LRN-121 shell allowlist validation (grep -Eq fragile → whole-string POSIX case) + seo-account-mgmt journal + contract

.claude/memory/learnings.md

@@ -1208,3 +1208,9 @@ rules:
 - **why it matters**: the final review is the last gate before merge; a wrong base hides real changes or invents fake ones. The ledger BASE is a task resume-map, not a merge-delta anchor.
 - **future application**: for ANY whole-branch/final review, derive base from `git merge-base <target> HEAD`, never a stored/remembered SHA. Sanity-check: does `git log BASE..HEAD` list ONLY this branch's commits, nothing foreign? Diff-stats differ between candidate bases → recorded one is stale, trust merge-base.
 - **cousin**: [[LRN-119]] (same GSC+CrUX build); SDD skill's own "never HEAD~1" warning (same base-selection bug class).
+
+## LRN-121 — Shell allowlist validation: `grep -Eq` is fragile; use a whole-string POSIX `case`
+- **pattern**: guarding a user-supplied label to shell-safe ASCII with `printf '%s' "$v" | grep -Eq '^[A-Za-z0-9._-]+$'` failed 3 adversarial gate passes in a row: (1) command-injection framing (label interpolated into an agent-composed Bash line); (2) parser differential — the guard pre-scanned argv for the literal token `--label` while the downstream `argparse` ALSO accepts `--label=v` and abbreviations (`--labe`, `allow_abbrev=True`), so those forms reached the parser unchecked; (3) `grep -q` matches PER LINE, so a label with an embedded newline (`ok\nrm -rf`) passes because its FIRST line matches. Fix = replace the whole mechanism, don't patch again: `_label_safe() ( LC_ALL=C; case "$1" in ''|[!A-Za-z0-9]*|*[!A-Za-z0-9._-]*) exit 1;; esac )` — POSIX `case`, whole-string, C-locale subshell. No grep (no per-line), no regex, no second grammar to differ from; a newline is just a non-allowed byte caught by `*[!...]*`; `LC_ALL=C` stops UTF-8 collation widening `[A-Za-z0-9]` to homoglyphs (U+FF11, Kelvin U+212A).
+- **why it matters**: three distinct bypasses of the SAME guard = the approach was wrong, not each patch. `grep`'s line-orientation + locale-sensitive ranges, plus argv-prescan-vs-real-parser grammar drift, are the three classic ways an allowlist "passes" a string it shouldn't. Whole-string `case` in C locale closes all three at once. These were defense-in-depth (downstream used `"$2"`/`"$@"`/JSON-key, never `sh -c`/`eval` → not exploitable in the real exec chain) — but the backstop still took a categorical rewrite, and 3 security-gate BLOCKs to get there.
+- **future application**: validate shell input WHOLE-STRING (`case` or bash `[[ =~ ]]`), never `grep -q` (per-line). Set `LC_ALL=C` for byte-wise ranges. A guard that pre-scans argv must be STRICTER than the downstream parser (reject `=`-joined/abbrev) or validate post-parse against the value the parser settled on. When a fix is bypassed twice → STOP patching, replace the mechanism (re-plan, not whack-a-mole).
+- **cousin**: [[LRN-119]] (fail-open engine this hardens), [[BDR-063]] (token store whose labels these guard), [[LRN-045]] (renaming-command leak-guard regexes — same charset-guard family).