A name nothing covers raised `KeyError` in four places and
`AmbiguousDatabaseError` in a fifth, so a caller had to catch both and neither
name said what happened. `UnknownDatabaseError` is all of them, exported from
`haiku.rag.store` beside the other errors.
It subclasses `KeyError`, since selecting by name is a lookup, and prints its
message plainly rather than quoted as a missing key. Both CLIs turn it into the
same clean exit they already gave the others.
`sources` was ignored beside a database path, so `sources=["nope"]` opened the
path and read as though the selection had been honoured. A path and a name
already conflict inside `DatabaseScope.resolve`; this is the same rule where a
caller can reach it.
`reader_for` returned itself before looking at the name, so an "alpha" client
answered `reader_for("beta")` with alpha's reader. A citation naming another
database would have been read from the wrong one.
Both paths decide through `clients_covering` now, so one database refuses a
wrong name the way a set already did.
The chat citation test said mounting lends the client without showing it; a
chat test shows it and the docstring points there.
`--database` and `--db` read as one word abbreviated but take different
things, a configured name against a filesystem path, and they sit in
different positions: `haiku-rag --db /path info` fails with `No such option`.
The CLI already says "database" to the user everywhere it prints one, so the
name it selects by should say the same. Unreleased, so no deprecation.
Also: without a reranker, raise `search.limit` with the number of databases
searched, since each contributes its best matches to a list truncated back
to the limit.
Two files of 1,405 and 814 lines become seven: scope resolution, lifecycle,
search, documents, expansion, citations and capabilities. `_config`, `_seed`
and the rest move to `helpers.py`, importable by the sandbox tests that share
them, and the package points VCR back at `tests/cassettes/multi_db/`.
