feat(agents): change prompt lookup for agent registry

What does this merge request do and why?

Related discussions:

• !1063 (comment 1990471075)
• Agents prompts for multiple/different models (#530 - closed)

Before

• Prompt definitions are stored either as specific/feature.yml or specific_feature/base.yml:
  • When specific/feature is specified, the definition is fetched as-is: specific/feature.yml
  • When specific_feature is specified (without a slash in the name), then base.yml is appended: specific_feature/base.yml

Now

• Prompt definitions are stored in a folder per feature: feature or specific/feature or very/specific/feature.
  • The name of yml files is either the name of the model for which the prompt is defined or base.yml: feature/codegemma.yml or specific/feature/base.yml or very/specific/feature/mistral.yml
  • When specific/feature is specified:
    • If the model name is passed via model metadata, then fetch specific/feature/model-name.yml
    • Otherwise, append base.yml: specific/feature/base.yml