fix(stacker): avoid duplicate stack deletions

[Majorfi]

Summary

• deduplicate child stack IDs before issuing delete requests
• exclude successfully deleted stacks from the in-memory snapshot while retaining failed or refused deletions
• treat already-gone stack responses as benign without reporting deletions that did not happen
• mask API keys in client initialization and user lookup errors
• add regression coverage for stack deletion, replacement, dry-run, and logging behavior

Testing

• go test ./...
• go vet ./...

Closes #80