merge(cycle-2): land Stream B — Plan B Phases 4+5+6 (session/manifest discipline)

4 commits from feature/cli-tail-stream-b-session-manifest:
- 2e41e0b refactor(cli): single canonical ParamsFile in session.rs (Phase 5)
- 7901c27 refactor(cli): Vault::after_manifest_change wrapper (Phase 4)
- 4b657e7 refactor(cli): batched purge in cmd_purge and cmd_trash_empty (Phase 6)
- c4777cc refactor(cli): apply simplify findings (Phases 4-6 polish)

Phase 4 complete: Vault::after_manifest_change wrapper funnels NINE manifest-
mutation sites (not 7 as the spec/notes flagged -- attach.rs add+detach,
import.rs LastPass, and trash.rs cmd_trash_empty all previously SKIPPED
refresh_groups_cache; the wrapper now refreshes them as a side-effect).
save_manifest was DROPPED entirely (rather than just demoted to pub(crate)
as the spec said) -- the simplify pass found no escape hatch was needed,
so the only path to write the manifest now goes through the wrapper.
Stronger than spec.

Phase 5 complete: single pub(crate) struct ParamsFile in session.rs at
module level with Serialize+Deserialize. Constructors for_new_vault and
to_kdf_params (simplify pass changed into_kdf_params(self) to
to_kdf_params(&self) for ergonomics). commands/init.rs uses
ParamsFile::for_new_vault. On-disk JSON schema verified BYTE-STABLE via
fixture-string round-trip test (session::tests::params_file_round_trips_current_layout
+ for_new_vault_produces_expected_shape) -- same fields, same ordering,
same rename_all placement. Existing vaults read with no migration.

Phase 6 complete: purge_item renamed purge_item_filesystem, mutates only
filesystem + manifest, returns Vec<String> of paths. cmd_purge and
cmd_trash_empty both follow after_manifest_change -> git_rm -> git add ->
git commit. New helpers::git_rm extracted to DRY the pattern. Strict
invariant locked: tests/basic_flows.rs::trash_empty_batches_into_one_commit
counts commits via git rev-list --count HEAD before/after and asserts
delta == 1. A 50-item trash empty now fires 3 git invocations, not 52.

Simplify polish (c4777cc): all 5 findings legitimate, none rationale-skipped:
- Dropped redundant save_manifest_raw escape hatch
- Value-vs-self ergonomic fix (to_kdf_params(&self))
- DRY git_rm helper
- TOCTOU pre-check dropped from purge_item_filesystem
- Comment trim

3-way merge with stream-a (3dd1e1b) and stream-c (e69b347) clean: git
auto-resolved commands/add.rs (stream-a prompt_or_flag changes interleaved
with stream-b after_manifest_change call at the manifest-mutation site).
Verified semantic correctness via post-merge cargo test.

Pre-merge checklist on tip c4777cc + post-merge verification:
- cargo test --workspace standalone: 260 tests, 0 failures
- cargo test --workspace post-merge: 281 tests, 0 failures
- cargo clippy --workspace --all-targets -- -D warnings: silent
- cargo build -p relicario-wasm --target wasm32-unknown-unknown: clean
- Independent fresh-subagent code review: APPROVE
- grep refresh_groups_cache crates/relicario-cli/src/: zero matches
  outside session.rs/helpers.rs (per spec done-criteria)
- grep struct ParamsFile crates/relicario-cli/src/: ONE match
  (per spec done-criteria)

Plan B COMPLETE. With Phase 3 (Stream A) merged at 3dd1e1b and Phases 7+8
(Stream C) merged at e69b347, all eight Plan B phases are now on main.

One nit deferred (per subagent review): trash empty partial-failure
recovery -- if git_rm fails after after_manifest_change succeeds,
manifest.enc is rewritten in-tree and items are removed from disk but
no commit is made. Pre-existing behavior was strictly worse (per-item
interleaved partial-commit risk); current state is a net improvement.
Tree-cleanup-on-failure belongs in a follow-up plan, not this PR.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-cli/src/commands/add.rs

@@ -36,8 +36,7 @@ pub fn cmd_add(kind: AddKind) -> Result<()> {
 
     vault.save_item(&item)?;
     manifest.upsert(&item);
-    vault.save_manifest(&manifest)?;
-    crate::refresh_groups_cache(vault.root(), &manifest);
+    vault.after_manifest_change(&manifest)?;
 
     let mut paths: Vec<String> = vec![
         format!("items/{}.enc", item.id.as_str()),