guardrails: silence the intentional SC2016 in prod-guard

shell-lint flagged the literal `$(` / backtick patterns the read-only classifier
searches for inside a command string. Expanding them is exactly what must not
happen — the check exists to refuse commands containing command substitution, so
the single quotes are the point. Same treatment and rationale as the existing
disables in render-secrets.sh and verify-centralis-render.sh.

Verified with the pinned shellcheck v0.11.0 (sha256 matching shell-lint.yml):
all 34 scripts in the tree clean. Hook behaviour unchanged — the classifier
test matrix still allows reads and asks on mutations and compound smuggling.

Also documents that ~/.claude/hooks/ carries a mirror of prod-guard.sh so
sessions started outside this repo are covered, and that the blanket ssh allow
rules were removed from global settings so the hook cannot be out-ranked.

.claude/hooks/README.md

@@ -54,12 +54,31 @@ fails toward the prompt rather than toward silent execution.
 
 ## lint-after-edit needs shellcheck on PATH
 
-It exits quietly when shellcheck is absent, so it is **inert until installed** — a
+It exits quietly when shellcheck is absent — a missing local tool must not break
-missing local tool must not break editing, and `shell-lint` CI is still the backstop.
+editing, and `shell-lint` CI is still the backstop. v0.11.0 (the version CI pins) is
-Install the same pinned version CI uses:
+installed at `~/.local/bin/shellcheck`; if you move machines, reinstall it:
 
 ```bash
 VER=v0.11.0
 curl -fsSL "https://github.com/koalaman/shellcheck/releases/download/${VER}/shellcheck-${VER}.linux.x86_64.tar.xz" \
   | tar -xJ --strip-components=1 -C ~/.local/bin "shellcheck-${VER}/shellcheck"
 ```
+
+Keep it matched to `shell-lint.yml`'s pin. A drifted shellcheck grows *new* warnings
+and fails CI on an unrelated push, which is why that workflow pins the version and
+its sha256 rather than using `apt-get install`.
+
+## The global copy
+
+`~/.claude/settings.json` runs `prod-guard.sh` from `~/.claude/hooks/` as well, so a
+session started **outside** this repo is covered too — otherwise the guard would be
+trivially bypassed by working from `~/Code`. That copy is a mirror; this one is
+canonical. If you change the classifier here, copy it over:
+
+```bash
+cp .claude/hooks/prod-guard.sh ~/.claude/hooks/prod-guard.sh
+```
+
+The blanket `Bash(ssh prod:*)` / `Bash(ssh beta:*)` allow rules were removed from
+global settings at the same time, so the hook is the only thing granting live-host
+access. If it is missing or broken, nothing allows the command and you get a prompt.

.claude/hooks/prod-guard.sh

@@ -55,6 +55,9 @@ is_readonly() {
   case "$stripped" in *'>'*) return 1 ;; esac
 
   # Command substitution can hide anything; refuse to reason about it.
+  # shellcheck disable=SC2016  # single quotes are the point: these are literal
+  # `$(` and backtick characters being searched for inside the command text, not
+  # expansions to perform. Expanding them here would defeat the check entirely.
   case "$cmd" in *'$('*|*'`'*) return 1 ;; esac
 
   while IFS= read -r seg; do