feat(agentic context): use DeepCodyHandler as default chat handler

[abeatrix]

CLOSE https://linear.app/sourcegraph/issue/CODY-5478

This commit changes the default chat handler to DeepCodyHandler. It also removes the ChatHandler import and updates the getAgent function to return a DeepCodyHandler instance when no specific model handler is found. The getAgentName function is also updated to consistently return DeepCodyAgentID.

Test plan

Agentic context is automatically included for all models