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>
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