refactor(kon): clean up storage and cloud-providers — remove unused deps, add safety docs

- Remove unused chrono and uuid dependencies from kon-storage
- Add schema versioning TODO to run_migrations() for pre-release safety
- Add doc comment and usage example to log_error() (uncalled public API)
- Add TODO to consolidate app_data_dir() with model_manager::dirs_path()
- Add safety comments and #[allow(deprecated)] to keystore set_var usage
- Add TODO for keyring crate migration in cloud-providers keystore
- Apply cargo fmt

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

crates/storage/Cargo.toml

@@ -12,9 +12,3 @@ sqlx = { version = "0.8", features = ["sqlite", "runtime-tokio"] }
 
 # Async runtime
 tokio = { version = "1", features = ["rt", "sync", "macros"] }
-
-# UUIDs for record IDs
-uuid = { version = "1", features = ["v4"] }
-
-# Timestamps
-chrono = { version = "0.4", features = ["serde"] }