refactor(cli): batched purge in cmd_purge and cmd_trash_empty (Plan B Phase 6)

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.

crates/relicario-cli/tests/basic_flows.rs

@@ -109,6 +109,72 @@ fn rm_restore_purge_cycle() {
     assert!(!String::from_utf8(out.stdout).unwrap().contains("target"));
 }
 
+#[test]
+fn trash_empty_batches_into_one_commit() {
+    let v = TestVault::init();
+
+    // Add 3 items.
+    for title in ["alpha", "bravo", "charlie"] {
+        let out = v.run(&[
+            "add", "login",
+            "--title", title,
+            "--username", "u",
+            "--password", "p",
+        ]);
+        assert!(out.status.success(), "add {title} failed");
+    }
+
+    // Soft-delete all 3.
+    for title in ["alpha", "bravo", "charlie"] {
+        let out = v.run(&["rm", title]);
+        assert!(out.status.success(), "rm {title} failed");
+    }
+
+    // Set retention to 0 days so the recently-trashed items become purgeable
+    // (should_purge: now - trashed_at > 0 * 86400 = 0).
+    let out = v.run(&["settings", "trash-retention", "--days", "0"]);
+    assert!(out.status.success(), "settings trash-retention failed");
+
+    // Brief sleep to ensure now > trashed_at by at least 1 second
+    // (otherwise should_purge returns false at strict-greater-than equality).
+    std::thread::sleep(std::time::Duration::from_secs(1));
+
+    // Count commits before.
+    let before = std::process::Command::new("git")
+        .args(["rev-list", "--count", "HEAD"])
+        .current_dir(v.path())
+        .output()
+        .unwrap();
+    let before_count: u32 = String::from_utf8(before.stdout).unwrap().trim().parse().unwrap();
+
+    // Run trash empty.
+    let out = v.run(&["trash", "empty"]);
+    assert!(out.status.success(), "trash empty failed: stderr={}",
+            String::from_utf8_lossy(&out.stderr));
+
+    // Count commits after.
+    let after = std::process::Command::new("git")
+        .args(["rev-list", "--count", "HEAD"])
+        .current_dir(v.path())
+        .output()
+        .unwrap();
+    let after_count: u32 = String::from_utf8(after.stdout).unwrap().trim().parse().unwrap();
+
+    assert_eq!(
+        after_count - before_count, 1,
+        "trash empty should fire exactly one commit; before={before_count} after={after_count}"
+    );
+
+    // The remaining `list --trashed` should be empty.
+    let out = v.run(&["list", "--trashed"]);
+    let stdout = String::from_utf8(out.stdout).unwrap();
+    let stderr = String::from_utf8(out.stderr).unwrap();
+    assert!(
+        !stdout.contains("alpha") && !stdout.contains("bravo") && !stdout.contains("charlie"),
+        "items still in trashed list: stdout={stdout} stderr={stderr}"
+    );
+}
+
 #[test]
 fn generate_random_and_bip39() {
     let dir = tempfile::TempDir::new().unwrap();