perf(sandbox): compile command-substitution regexps once

hasCommandSubstitution recompiled its three regexps with regexp.MustCompile
on every call, and ValidateArgs calls it once per argument — so validating
an N-arg command compiled 3N patterns. Hoist them to a package-level slice
compiled once at init.

Behavior is unchanged (same patterns); covered by the existing
TestScriptValidator_ValidateArgs / ValidateAll tests.

Benchmark, ValidateArgs with 8 args (-benchmem):
  before: 16374 ns/op, 40142 B/op, 505 allocs/op
  after:   2092 ns/op,    32 B/op,   1 allocs/op

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/sandbox/validator.go

@@ -217,15 +217,18 @@ func (v *ScriptValidator) hasShellOperators(s string) bool {
 	return false
 }
 
+// Command-substitution patterns, compiled once. hasCommandSubstitution runs
+// per argument in ValidateArgs, so recompiling these on every call wasted work
+// proportional to the argument count.
+var commandSubstitutionPatterns = []*regexp.Regexp{
+	regexp.MustCompile(`\$\([^)]+\)`),   // $(command)
+	regexp.MustCompile("`[^`]+`"),       // `command`
+	regexp.MustCompile(`\$\{[^}]*\$\(`), // ${...$(command)
+}
+
 // hasCommandSubstitution checks for command substitution patterns
 func (v *ScriptValidator) hasCommandSubstitution(s string) bool {
-	patterns := []*regexp.Regexp{
-		regexp.MustCompile(`\$\([^)]+\)`),   // $(command)
-		regexp.MustCompile("`[^`]+`"),       // `command`
-		regexp.MustCompile(`\$\{[^}]*\$\(`), // ${...$(command)
-	}
-
-	for _, p := range patterns {
+	for _, p := range commandSubstitutionPatterns {
 		if p.MatchString(s) {
 			return true
 		}

internal/sandbox/validator_test.go

@@ -359,3 +359,12 @@ func containsHelper(s, substr string) bool {
 	}
 	return false
 }
+
+func BenchmarkValidateArgs(b *testing.B) {
+	v := NewScriptValidator()
+	args := []string{"--input", "file.txt", "--name", "report 2024", "--out", "/tmp/x", "--verbose", "--limit=50"}
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_ = v.ValidateArgs(args)
+	}
+}