refactor: restructure project into src layout with proper packaging

Split monolithic transcribe.py into focused modules under
src/cohere_transcribe/ (model, vad, stream, cli), move tests into
tests/, add hatchling build system and CLI entry point, remove
unused shell.nix and main.py.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -8,3 +8,7 @@ wheels/
 
 # Virtual environments
 .venv
+
+# Nix
+.direnv/
+result