storage: bare store init/delete and remotesrv DBCache

InitStore writes a bare NBS store (LoadDoltDB Format_DOLT + WriteEmptyRepo)
with partial-failure cleanup; DeleteStore guards against paths escaping the
configured repos root; RepoDiskPath lays out <root>/~<owner>/<name>.

Cache implements remotesrv.DBCache over an injected RepoLookup (no db import,
no push-to-create): Get resolves via core.ParseRepoPath, memoizes one
nbs.NewLocalStore per disk path, never creates directories; Evict/Close for
delete and shutdown. Package doc records the LocalFilesysWithWorkingDir
sealed-URL requirement proven by the Phase-0 spike.

Unit tests cover store validity (reopen, main branch, initial commit),
partial-failure cleanup, root-escape guard, and cache hit/miss/memoize/evict.
Spike test untouched and still green.