refactor(cli): Vault::after_manifest_change wrapper (Plan B Phase 4)

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.

crates/relicario-cli/src/helpers.rs

@@ -126,6 +126,24 @@ pub fn groups_cache_path(vault_dir: &Path) -> PathBuf {
     vault_dir.join(".relicario").join("groups.cache")
 }
 
+/// Collect all non-empty group names from the manifest and write them to the
+/// plaintext `groups.cache` file so shell completion can enumerate `--group`
+/// candidates without prompting for the vault passphrase.
+///
+/// Failures are silently swallowed — a missing cache is merely a UX degradation,
+/// not a correctness problem.
+pub fn refresh_groups_cache(vault_dir: &Path, manifest: &relicario_core::Manifest) {
+    let mut set = std::collections::BTreeSet::<String>::new();
+    for entry in manifest.items.values() {
+        if let Some(g) = entry.group.as_ref() {
+            if !g.is_empty() {
+                set.insert(g.clone());
+            }
+        }
+    }
+    let _ = write_groups_cache(vault_dir, &set);
+}
+
 /// Write the sorted set of group names to `<vault_dir>/.relicario/groups.cache`,
 /// one name per line. In debug builds, setting `RELICARIO_NO_GROUPS_CACHE`
 /// suppresses the write (developer debugging tool). In release builds the env