feat(kon): add cloud-providers crate — BYOK stubs, keystore

- Keystore stub using environment variables (keyring integration deferred)
- Minimal crate ready for cloud STT provider implementations post-Stage 1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/cloud-providers/src/lib.rs

@@ -1,2 +1,3 @@
-// kon-cloud-providers: BYOK cloud STT provider implementations
-// and OS keychain API key storage.
+pub mod keystore;
+
+pub use keystore::{retrieve_api_key, store_api_key};