Add Vertigo Panda alias to UNC6384 threat-actor entry

[adulau]

Motivation

• Issue PRC-Nexus threat actor Vertigo Panda, UNC6384 - close as patterns to Mustang Panda #1102 asked to add Vertigo Panda; the existing UNC6384 entry corresponds to the same reporting, so adding an alias avoids creating a duplicated threat actor and improves discoverability by alternate naming.

Description

• Updated clusters/threat-actor.json to modify the UNC6384 entry by adding Vertigo Panda to meta.synonyms and updating the description to note it is "also tracked as Vertigo Panda", and ensured the GTIG reference is present.

Testing

• Ran python -m json.tool clusters/threat-actor.json which succeeded, and executed ./validate_all.sh which performed the full schema validation successfully but exited non-zero at the end due to the repository policy check that detects local changes during the validation pipeline.

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Codex Task