feat(kebab-store-sqlite): add NotIndexed typed error (fb-27)

New `SqliteStore::open_existing` API + `NotIndexed` signal for the
missing-DB case. kebab-app re-exports the type via its `error_signal`
module so kebab-cli's `error_classify` can map it to
`error.v1 { code: "not_indexed" }`.

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

crates/kebab-store-sqlite/tests/not_indexed.rs

@@ -0,0 +1,19 @@
+//! Signal test: `SqliteStore::open_existing` emits `NotIndexed` when the DB
+//! file is absent.
+
+use kebab_store_sqlite::{NotIndexed, SqliteStore};
+
+#[test]
+fn not_indexed_signal_emitted_when_db_missing() {
+    let dir = tempfile::tempdir().unwrap();
+    let nonexistent_db = dir.path().join("does-not-exist.sqlite");
+    let res = SqliteStore::open_existing(&nonexistent_db);
+    let err = match res {
+        Ok(_) => panic!("opening a missing DB should fail"),
+        Err(e) => e,
+    };
+    let signal = err
+        .downcast_ref::<NotIndexed>()
+        .expect("missing DB error should downcast to NotIndexed");
+    assert_eq!(signal.expected, nonexistent_db.to_string_lossy().as_ref());
+}