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/keystore.rs

@@ -0,0 +1,14 @@
+/// Store an API key in the OS keychain.
+/// Stub implementation using environment variables until keyring crate is added.
+pub fn store_api_key(provider: &str, key: &str) {
+    std::env::set_var(
+        format!("KON_API_KEY_{}", provider.to_uppercase()),
+        key,
+    );
+}
+
+/// Retrieve an API key from the OS keychain.
+/// Stub implementation using environment variables until keyring crate is added.
+pub fn retrieve_api_key(provider: &str) -> Option<String> {
+    std::env::var(format!("KON_API_KEY_{}", provider.to_uppercase())).ok()
+}

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};