Adds the canonical post-mutation funnel: save_manifest_raw + groups.cache
refresh in one method. Converts nine commands/*.rs mutation callsites from
the manual save_manifest + refresh_groups_cache pair to a single
vault.after_manifest_change(&manifest)?. save_manifest renamed to
save_manifest_raw (pub(crate)) so future commands cannot accidentally
bypass the cache refresh. Four of the nine sites (attach.rs add/detach,
import.rs LastPass, trash.rs cmd_trash_empty's per-item save) previously
skipped the cache refresh — the wrapper fixes them. refresh_groups_cache
moves from main.rs to helpers.rs so the read-side warmup callers in
get.rs/list.rs still reach it.
