fix(lint): resolve all golangci-lint errors

- Mark unused ctx parameter with underscore in getRepoRootForWorktree
- Replace exec.Command("test", "-d") with os.Stat for directory check
- Handle file.Close() errors properly in compact.go and migrate_tombstones.go
- Explicitly ignore cleanup errors with _ assignment

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/git/gitdir.go

@@ -2,6 +2,7 @@ package git
 
 import (
 	"fmt"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
@@ -106,7 +107,7 @@ func GetMainRepoRoot() (string, error) {
 		commonDir := getGitDirNoError("--git-common-dir")
 		if commonDir != "" {
 			// Validate that commonDir exists
-			if _, err := exec.Command("test", "-d", commonDir).Output(); err == nil {
+			if info, err := os.Stat(commonDir); err == nil && info.IsDir() {
 				return filepath.Dir(commonDir), nil
 			}
 		}