chore(import): harden NetBox import workflow input handling

[ggfevans]

Parent

Part of #1276

Summary

Harden import-netbox.yml input handling to prevent brittle shell behavior and improve reliability for workflow_dispatch runs.

Problem

Workflow-dispatch inputs are currently interpolated into shell flags in ways that can break on special characters/whitespace.

Acceptance Criteria

• Build CLI args using safe shell patterns (array-based, robust quoting).
• Validate/sanitize user inputs (vendor/slug) before use.
• Ensure generated branch names are safe and deterministic.
• Add failure messages that clearly identify invalid input.
• Keep dry-run and normal modes behaviorally equivalent aside from write operations.

Notes

Focus on reliability and operator safety, not feature expansion.