feat(kebab-config): add ConfigInvalid typed error (fb-27)

Wraps every error path in `Config::from_file` (read failure, TOML parse,
validation) so downstream callers can `downcast_ref::<ConfigInvalid>()`
to build the `error.v1` wire record. kebab-app re-exports the type via
its `error_signal` module.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-app/src/error_signal.rs

@@ -11,5 +11,5 @@
 pub use crate::doctor_signal::{DoctorUnhealthy, NoHitSignal, RefusalSignal};
 
 pub use kebab_llm_local::LlmError;
-// pub use kebab_config::ConfigInvalid;        // wired in Task 2
+pub use kebab_config::ConfigInvalid;        // wired in Task 2
 // pub use kebab_store_sqlite::NotIndexed;     // wired in Task 3