feat(lib): extract shell command-substitution parser to shared lib

Extract the `extractCommandSubstitutions` function originally
introduced in scripts/hooks/gateguard-fact-force.js (PR #1853
round 2) into scripts/lib/shell-substitution.js so other PreToolUse
hooks can reuse the same single-quote-aware, double-quote-aware,
nested-subshell-aware parser without duplicating it.

No behavior change in this commit — the function body is copied
verbatim and exposed via `module.exports`. The next commit wires it
into scripts/hooks/pre-bash-dev-server-block.js to close that hook's
own subshell-bypass holes.

gateguard-fact-force.js still defines its own private copy of the
function; consolidating both call sites onto this shared lib is a
follow-up worth doing once this PR lands, but is intentionally out
of scope here to keep the diff focused on the dev-server-block fix.

scripts/lib/shell-substitution.js

@@ -0,0 +1,109 @@
+'use strict';
+
+/**
+ * Extract executable command-substitution bodies from a shell line.
+ *
+ * Single quotes are literal, so substitutions inside them are ignored;
+ * double quotes still permit substitutions, so those bodies are scanned
+ * before quoted text is stripped. Returns each substitution body plus
+ * any nested substitutions discovered recursively.
+ *
+ * Originally introduced in scripts/hooks/gateguard-fact-force.js
+ * (PR #1853 round 2). Extracted to a shared lib so other PreToolUse
+ * hooks that need the same "scan inside `$(...)` and backticks"
+ * behavior can reuse it without duplicating the parser.
+ *
+ * @param {string} input
+ * @returns {string[]}
+ */
+function extractCommandSubstitutions(input) {
+  const source = String(input || '');
+  const substitutions = [];
+  let inSingle = false;
+  let inDouble = false;
+
+  for (let i = 0; i < source.length; i++) {
+    const ch = source[i];
+    const prev = source[i - 1];
+
+    if (ch === '\\' && !inSingle) {
+      i += 1;
+      continue;
+    }
+
+    if (ch === "'" && !inDouble && prev !== '\\') {
+      inSingle = !inSingle;
+      continue;
+    }
+
+    if (ch === '"' && !inSingle && prev !== '\\') {
+      inDouble = !inDouble;
+      continue;
+    }
+
+    if (inSingle) {
+      continue;
+    }
+
+    if (ch === '`') {
+      let body = '';
+      i += 1;
+      while (i < source.length) {
+        const inner = source[i];
+        if (inner === '\\') {
+          body += inner;
+          if (i + 1 < source.length) {
+            body += source[i + 1];
+            i += 2;
+            continue;
+          }
+        }
+        if (inner === '`') {
+          break;
+        }
+        body += inner;
+        i += 1;
+      }
+      if (body.trim()) {
+        substitutions.push(body);
+        substitutions.push(...extractCommandSubstitutions(body));
+      }
+      continue;
+    }
+
+    if (ch === '$' && source[i + 1] === '(') {
+      let depth = 1;
+      let body = '';
+      i += 2;
+      while (i < source.length && depth > 0) {
+        const inner = source[i];
+        if (inner === '\\') {
+          body += inner;
+          if (i + 1 < source.length) {
+            body += source[i + 1];
+            i += 2;
+            continue;
+          }
+        }
+        if (inner === '(') {
+          depth += 1;
+        } else if (inner === ')') {
+          depth -= 1;
+          if (depth === 0) {
+            break;
+          }
+        }
+        body += inner;
+        i += 1;
+      }
+      if (body.trim()) {
+        substitutions.push(body);
+        substitutions.push(...extractCommandSubstitutions(body));
+      }
+    }
+  }
+
+  return substitutions;
+}
+
+module.exports = { extractCommandSubstitutions };