`storage: { type: 'filesystem', path: '…' }` is a widely-used, doc-promoted
config shape, but the 8.0 storage refactor dropped top-level `path` from the
root-directory resolution chain — so it was silently ignored and every such
brain wrote to the default `./brainy-data` instead. A consumer upgrading with
`storage: { path: './my-data' }` would have had their data quietly relocated.
Restores `path` as a first-class alias for `rootDirectory` (it already worked
nested under `options.path`; now it works top-level too), adds it to the
StorageOptions / BrainyConfig.storage types, and pins every accepted spelling
(rootDirectory, path, options.rootDirectory, options.path, default) in a unit
test of the resolution chain.
