test: cover absolutizePaths, lastID, runCheckMigrations

Adds focused unit tests for previously-uncovered cli helpers:

- TestAbsolutizePaths covers the path-vararg helper's empty,
  absolute, and relative branches; complements the existing
  TestAbsolutizeImageRegisterPaths.
- TestLastID is table-driven across nil/empty/sorted/unsorted/
  duplicates/negative inputs.
- TestRunCheckMigrations* exercises every Compatibility branch
  (compatible / migrations needed / incompatible / inspect
  error) by stubbing bangerdExit and pointing the layout at a
  temp-dir DB seeded directly with the schema_migrations table.
- TestNewBangerdCommandSubcommands pins the flag set against
  accidental drift.

Lifts internal/cli coverage from 71% to 76% combined.

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

internal/cli/cli_test.go

@@ -737,6 +737,50 @@ func TestAbsolutizeImageRegisterPaths(t *testing.T) {
 	}
 }
 
+func TestAbsolutizePaths(t *testing.T) {
+	tmp := t.TempDir()
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("Getwd: %v", err)
+	}
+	if err := os.Chdir(tmp); err != nil {
+		t.Fatalf("Chdir: %v", err)
+	}
+	t.Cleanup(func() { _ = os.Chdir(wd) })
+
+	empty := ""
+	abs := "/already/absolute/path"
+	rel1 := filepath.Join("a", "b")
+	rel2 := "./c/d"
+
+	if err := absolutizePaths(&empty, &abs, &rel1, &rel2); err != nil {
+		t.Fatalf("absolutizePaths: %v", err)
+	}
+
+	if empty != "" {
+		t.Errorf("empty value mutated: %q", empty)
+	}
+	if abs != "/already/absolute/path" {
+		t.Errorf("absolute value mutated: %q", abs)
+	}
+	if !filepath.IsAbs(rel1) {
+		t.Errorf("rel1 not absolutized: %q", rel1)
+	}
+	if !filepath.IsAbs(rel2) {
+		t.Errorf("rel2 not absolutized: %q", rel2)
+	}
+	// Sanity: relative paths should land under tmp.
+	if !strings.HasPrefix(rel1, tmp) {
+		t.Errorf("rel1 = %q, want prefix %q", rel1, tmp)
+	}
+}
+
+func TestAbsolutizePathsNoArgs(t *testing.T) {
+	if err := absolutizePaths(); err != nil {
+		t.Fatalf("absolutizePaths() with no args: %v", err)
+	}
+}
+
 func TestPrintImageListTableShowsRootfsSizes(t *testing.T) {
 	rootfs := filepath.Join(t.TempDir(), "rootfs.ext4")
 	if err := os.WriteFile(rootfs, nil, 0o644); err != nil {