Renames purge_item to purge_item_filesystem — body becomes filesystem-only
(remove item.enc, remove attachments/<id>/, manifest.remove). Returns the
relative paths it removed. cmd_purge and cmd_trash_empty accumulate the
paths and fire ONE git rm + ONE git add + ONE git commit per invocation.
A 50-item trash empty now produces 3 git subprocesses regardless of N
(was N+2). New regression test trash_empty_batches_into_one_commit asserts
the one-commit invariant via git rev-list --count.
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.
Promotes ParamsFile to a module-level pub(crate) struct with both Serialize
and Deserialize derives. for_new_vault() constructor + into_kdf_params()
inversion replace the two-definition split between commands/init.rs (write)
and session.rs read_params (read). On-disk JSON format unchanged — fixture
test asserts round-trip with the current params.json layout.
