parsePromptIdentifier

Parses a prompt identifier string into its component parts. Supported formats:

• "name" → ("-", "name", "latest")
• "owner/name" → ("owner", "name", "latest")
• "name:commit" → ("-", "name", "commit")
• "owner/name:commit" → ("owner", "name", "commit")
• identifier: the prompt identifier string Returns: a [PromptIdentifier] with owner, repo, and commit fields
• Throws IllegalArgumentException: if the identifier is blank or has an invalid format