bug: File syntax for prompt field broken for sub agents

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] 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](https://kiro.dev/docs/cli/custom-agents/configuration-reference/#file-uri-prompt)

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

### Environment

```yaml

```