fix: resolve golangci-lint warnings

- Handle ignored errors with explicit _ assignment (errcheck)
- Add #nosec comments for false positive G304/G204 warnings (gosec)
- Fix misspelling: cancelled -> canceled

🤖 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