refactor(cli): apply simplify findings (Plan B Phases 4-6 polish)

- session.rs: drop save_manifest_raw — its only caller was
  after_manifest_change itself; the pub(crate) advertised the exact
  bypass-the-cache-refresh footgun the wrapper exists to eliminate.
  Inline the encrypt + atomic_write pair.
- session.rs: into_kdf_params(self) → to_kdf_params(&self). Body just
  copies three u32s; the consume-self had no ownership benefit and
  forced the round-trip test to rebuild a ParamsFile field-by-field.
- helpers.rs: add git_rm(repo, paths, context) wrapper around git_run
  + the load-bearing --ignore-unmatch flag. Replaces two near-identical
  three-line "build rm_args, extend, git_run" blocks in trash.rs.
- trash.rs: purge_item_filesystem drops the if x.exists() pre-checks
  (TOCTOU + redundant stat per item per trash-empty iteration). Uses
  ErrorKind::NotFound swallow on remove_file/remove_dir_all instead.
- basic_flows.rs: trim trash_empty_batches_into_one_commit's sleep
  comment to just the WHY.

crates/relicario-cli/src/helpers.rs

@@ -86,6 +86,16 @@ pub fn git_run(repo: &Path, args: &[&str], context: &str) -> Result<()> {
     Ok(())
 }
 
+/// Stage `paths` for removal in one `git rm -rf --ignore-unmatch` invocation.
+/// `--ignore-unmatch` is load-bearing: a previous partial-write crash can
+/// leave the manifest entry without the corresponding `items/<id>.enc` on
+/// disk, and we want the rm to succeed regardless.
+pub fn git_rm(repo: &Path, paths: &[String], context: &str) -> Result<()> {
+    let mut args: Vec<&str> = vec!["rm", "-rf", "--ignore-unmatch"];
+    args.extend(paths.iter().map(String::as_str));
+    git_run(repo, &args, context)
+}
+
 /// Format a Unix-seconds timestamp as an ISO-8601 UTC string.
 /// Audit M11: replaces the old `now_iso8601` helper that actually returned
 /// a numeric string.