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>