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>

pyproject.toml

@@ -1,7 +1,7 @@
 [project]
-name = "cohere"
+name = "cohere-transcribe"
 version = "0.1.0"
-description = "Add your description here"
+description = "Live speech transcription using Cohere ASR"
 readme = "README.md"
 requires-python = ">=3.14"
 dependencies = [
@@ -15,3 +15,13 @@ dependencies = [
     "torch>=2.12.0",
     "transformers>=5.9.0",
 ]
+
+[project.scripts]
+cohere-transcribe = "cohere_transcribe.cli:main"
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.backends"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/cohere_transcribe"]