[idea] Move agent support into plugin interface

[airtonix]

idea / problem

There's going to be an constant avalanche of people requesting support for harness x.y.z

I would suggest that the plugin interface accommodate registering support for harnesses, this moves maintenance burden for existing and new harnesses onto the community.

requested change

implement changes to plugin interface and integration so that integrating with any harness is done via external plugin modules instead of changes to core

why you want this

so you can focus on what matters

[airtonix]

evidence of the deluge:

• [idea] Support junie-cli #2700
• Jcode integration — complete, ready to pull #2740
• Add BitFun CLI to the agent catalog #2732
• Add native session restore for Kiro CLI #2512
• [idea] Zeroclaw integration pr to support herdr out of the box #811
• [idea] Integration for Deep Agents Code (dcode) #2689

and possibly billions others you haven't seen yet

agent adjacent:

• Proposal: a structured chat API for Herdr #2677