(feat) customizable subagent config and system env prompt

[pschiel]

Adds config for subagents.

Example (agent config)

system:
  model_id: false
  env: false
  files: false
agents:
  title: none
  explore: explore-lmstudio
  compaction: compaction-lmstudio

System prompt options

system.model_id - when false, don't add model info in system prompt
system.env - when false, don't add environment info in system prompt
system.files - when false, don't add files section in system prompt

Internal subagent options (ovverides small_model)

agents.title - custom title agent. when set to none, will not use LLM summary, but first 50 chars from the message
agents.explore - custom explore agent
agents.compaction - custom compaction agent