bug: File syntax for prompt field broken for sub agents

Checks

• I have searched github.com/aws/amazon-q-developer-cli/issues and there are no duplicates of my issue
• I have run q doctor in the affected terminal session
• I have run q restart and replicated the issue again

Operating system

mac

Expected behaviour

I want to define an agent which can be used both as an agent and a sub agent and uses the file syntax feature for its prompt

Actual behaviour

This isn't possible because the file is looked up from a different relative path based on whether it is launched as an agent or a sub agent. This means only full paths can work. Full paths make the agent definition far less portable.

Steps to reproduce

Make an agent like the one given as an example in the documentation

try to launch it as a sub agent and it will fail to load context from its prompt file

Environment

Inizia con l’esempio di prompt file-URI nella documentazione di riferimento collegata per la configurazione di custom-agents e riproducilo una volta come agente e una volta come sub-agente. Traccia il punto in cui ogni avvio risolve il file di prompt, quindi verifica che la stessa definizione portabile di relative-path venga caricata in entrambe le modalità senza richiedere un percorso completo.