clippy flagged "you should consider adding a `Default` implementation for
`CubeAi`". Add the impl delegating to `new()`.
Also: impl block methods had been accidentally pulled out of `impl CubeAi`
into module scope during a prior edit; rewrite the impl block with all
methods (ingest_trace, classify, suggest, ingest_and_suggest, store)
inside it so it compiles.
Verification: all 5 cubeai tests pass, clippy clean.
Co-Authored-By: Hermes Agent
