♻️(agent) simplify Deepgram config and support Kyutai

The previous attempt to make the Deepgram configuration extensible
introduced unnecessary complexity for a very limited use case and
made it harder to add new STT backends.

Refactor to a deliberately simple and explicit design with minimal
cognitive overhead. Configuration is now fully driven by environment
variables and provides enough flexibility for ops to select and
parameterize the STT backend.

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to
 
 ## [Unreleased]
 
+- ✨(agent) support Kyutai client for subtitle
+
 ## [1.1.0] - 2025-12-22
 
 ### Added