Add support to custom subagents

[srtab]

The DAIV agent supports the declaration and spawning of specialized sub-agents, but does not support custom sub-agents on a per-repository basis.

The implementation could include a skills definition in front matter format with the necessary parameters to define the sub agent. The content of the Markdown file would define the sub agent's system prompt.

[srtab]

DeepAgents is already working on something aligned with this: langchain-ai/deepagents#916