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>

cmd/bd/migrate_tombstones.go

@@ -128,7 +128,7 @@ Examples:
 					existingTombstones[issue.ID] = true
 				}
 			}
-			file.Close()
+			_ = file.Close()
 		}
 
 		// Determine which deletions need migration