refactor(llm): move transcription module into src/llm/ (#1559)

* refactor(llm): move transcription module into src/llm/

Transcription is an LLM capability (Whisper, Chat Completions audio).
Move it from a top-level module into src/llm/transcription/ to reflect
this, and update all references across the codebase.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* style: fix rustfmt formatting after module move

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

src/llm/mod.rs

@@ -35,6 +35,7 @@ mod rig_adapter;
 pub mod session;
 pub mod smart_routing;
 mod token_refreshing;
+pub mod transcription;
 
 #[cfg(test)]
 mod codex_test_helpers;