fix: reject unproxyable commands like cd with clear error

cd must run in the parent shell to change directory. Running it through
snip silently succeeded (exit 0) but had no effect, confusing AI agents
into thinking the directory changed.

Now snip exits with code 1 and a clear error message for commands that
cannot be proxied (cd, source, .).

Fixes #18

Author: edouard-claude

internal/cli/cli.go

@@ -38,6 +38,13 @@ func Run(args []string) int {
 	command := remaining[0]
 	cmdArgs := remaining[1:]
 
+	// Commands that cannot be proxied: they must run in the parent shell
+	// to have any effect. Running them in a subprocess is a silent no-op.
+	if unproxyableReason(command) != "" {
+		fmt.Fprintf(os.Stderr, "snip: %s cannot be proxied (%s)\n", command, unproxyableReason(command))
+		return 1
+	}
+
 	// Built-in commands
 	switch command {
 	case "init":
@@ -171,6 +178,18 @@ Examples:
 	fmt.Printf(usage, version)
 }
 
+// unproxyableReason returns a human-readable reason if the command cannot be
+// proxied through an external process, or "" if it can.
+func unproxyableReason(command string) string {
+	switch command {
+	case "cd":
+		return "it must run in the parent shell to change directory"
+	case "source", ".":
+		return "it must run in the parent shell to modify the environment"
+	}
+	return ""
+}
+
 // Version returns the current version string.
 func Version() string {
 	return version

internal/cli/cli_test.go

@@ -0,0 +1,33 @@
+package cli
+
+import "testing"
+
+func TestUnproxyableCommands(t *testing.T) {
+	tests := []struct {
+		command string
+		want    bool
+	}{
+		{"cd", true},
+		{"source", true},
+		{".", true},
+		{"git", false},
+		{"go", false},
+		{"export", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.command, func(t *testing.T) {
+			got := unproxyableReason(tt.command) != ""
+			if got != tt.want {
+				t.Errorf("unproxyableReason(%q) returned %q, wantBlocked=%v", tt.command, unproxyableReason(tt.command), tt.want)
+			}
+		})
+	}
+}
+
+func TestRunRejectsCd(t *testing.T) {
+	code := Run([]string{"snip", "cd", "/tmp"})
+	if code != 1 {
+		t.Errorf("Run(cd) = %d, want 1", code)
+	}
+}

internal/engine/executor.go

@@ -25,7 +25,6 @@ var shellBuiltins = map[string]bool{
 	"source":   true,
 	"alias":    true,
 	"unalias":  true,
-	"cd":       true,
 	"eval":     true,
 	"set":      true,
 	"shopt":    true,