chore: address re-review findings from PR #147

[chmmou]

Re-review fixes for the sessions delete slice (#60 / PR #147):

• S1: internal/session/store.go no longer carries a second package-doc comment that duplicated/narrowed doc.go; the persistence prose moved onto the Store type and doc.go is now the sole authoritative package doc.
• S2: sessions delete now prints a "cleared the local cache" note on the non-unknown_session failure path, so a failed-but-partially-applied state is not silent without --verbose.
• N1: session.Client.Delete guards against a nil response from a misbehaving Caller.
• N2: sessions delete --help states the global --yes flag has no effect.