🏗️ 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-store-sqlite/tests/not_indexed.rs

@@ -17,3 +17,11 @@ fn not_indexed_signal_emitted_when_db_missing() {
         .expect("missing DB error should downcast to NotIndexed");
     assert_eq!(signal.expected, nonexistent_db.to_string_lossy().as_ref());
 }
+
+#[test]
+fn open_existing_does_not_create_missing_db() {
+    let dir = tempfile::tempdir().unwrap();
+    let nonexistent_db = dir.path().join("does-not-exist.sqlite");
+    let _ = SqliteStore::open_existing(&nonexistent_db);
+    assert!(!nonexistent_db.exists(), "open_existing must NOT create the file");
+}