feat: add --update mode, --meta/--meta-plugin flags, streaming diff

- Add --update mode to modify tags and metadata for existing items by ID
- Add --meta key=value flag to set metadata during save/update
- Add --meta key (bare) to delete metadata keys or filter by existence
- Add --meta-plugin/-M name:{json} flag for plugin options via CLI
- Env meta plugin now uses options from --meta-plugin instead of only env vars
- Stream decompressed content to diff via /dev/fd pipes (no temp files)
- Wire --list-format CLI arg to settings (was parsed but ignored)
- Allow --info to accept tags (was restricted to numeric IDs only)
- Change DB meta filtering to HashMap<String, Option<String>> for exact match + key existence
- Fix fcntl error checking in diff pre_exec
- Fix README inaccuracies (delete by tag, nonexistent --digest flag, meta plugin key names)

src/modes/list.rs

@@ -179,7 +179,12 @@ pub fn mode_list(
     }
 
     let item_service = ItemService::new(data_path.clone());
-    let items_with_meta = item_service.list_items(conn, tags, &std::collections::HashMap::new())?;
+    let meta_filter: std::collections::HashMap<String, Option<String>> = settings
+        .meta
+        .iter()
+        .map(|(k, v)| (k.clone(), v.clone()))
+        .collect();
+    let items_with_meta = item_service.list_items(conn, tags, &meta_filter)?;
 
     let output_format = crate::modes::common::settings_output_format(settings);