🏗️ refactor(kebab-store-sqlite): harden open_existing against silent create (fb-27)

Replace `path.exists()` + `Connection::open` (which silently CREATEs on
race) with `Connection::open_with_flags` using READ_WRITE|URI but NOT
CREATE. SQLite surfaces `SQLITE_CANTOPEN` for missing files; we wrap as
NotIndexed { found: None } as before.

Adds open_existing_does_not_create_missing_db regression test pinning
the no-side-effect invariant.

Also documents read-only intent on open_existing, the format contract
on NotIndexed.found, and removes scaffolding comments from kebab-app
error_signal that are no longer load-bearing.

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_store_sqlite::NotIndexed;         // wired in Task 3
+pub use kebab_config::ConfigInvalid;
+pub use kebab_store_sqlite::NotIndexed;