fix: drop /root/repo fallback in vm exec for unbound VMs

vm exec defaulted execGuestPath to /root/repo whenever the VM had no
recorded workspace, so running it against a plain VM (one that never
had vm workspace prepare / vm run ./repo) blew up with
'cd: /root/repo: No such file or directory' — surfaced via the login
shell's mise activate hook because bash -lc sources profile.d before
the explicit cd. Now auto-cd only fires when --guest-path is passed
or the VM actually has a workspace recorded; otherwise the command
runs from root's home. Mise wrapping unchanged — without a .mise.toml
it's a no-op.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/cli/vm_exec_test.go

@@ -0,0 +1,35 @@
+package cli
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestBuildVMExecScriptWithGuestPath(t *testing.T) {
+	got := buildVMExecScript("/root/repo", []string{"make", "test"})
+	want := "cd '/root/repo' && if command -v mise >/dev/null 2>&1; then mise exec -- 'make' 'test'; else 'make' 'test'; fi"
+	if got != want {
+		t.Fatalf("buildVMExecScript with path:\n got:  %q\n want: %q", got, want)
+	}
+}
+
+func TestBuildVMExecScriptWithoutGuestPath(t *testing.T) {
+	got := buildVMExecScript("", []string{"whoami"})
+	want := "if command -v mise >/dev/null 2>&1; then mise exec -- 'whoami'; else 'whoami'; fi"
+	if got != want {
+		t.Fatalf("buildVMExecScript without path:\n got:  %q\n want: %q", got, want)
+	}
+	if strings.Contains(got, "cd ") {
+		t.Fatalf("expected no cd when guestPath is empty, got: %q", got)
+	}
+}
+
+func TestBuildVMExecScriptShellQuotesPathWithSpaces(t *testing.T) {
+	got := buildVMExecScript("/tmp/with space", []string{"echo", "a b"})
+	if !strings.Contains(got, "cd '/tmp/with space'") {
+		t.Fatalf("expected guest path to be shell-quoted, got: %q", got)
+	}
+	if !strings.Contains(got, "mise exec -- 'echo' 'a b'") {
+		t.Fatalf("expected command args to be shell-quoted, got: %q", got)
+	}
+}