🐛🧪 cli, vanth: Fix SQLite store bug and add CLI tests

- Modified `Sqlite::get_all_of_ty` to return stored rows instead of empty results.
- Added `Send` requirement to `Backend` trait.
- Implemented `ContentHash::hex` method for hexadecimal representation.
- Added integration test for CLI `write` and `get` commands with `run_vanth` helper.
- Configured CLI tracing to log to stderr instead of stdout.
- Added `serde` dependency to `cli` crate for test struct serialization.
- Modified `handle_write` to print content hash after successful storage.

crates/cli/src/cli.rs

@@ -137,6 +137,8 @@ fn handle_write(args: &WriteArgs) {
         eprintln!("Error writing to store: {:?}", e);
         process::exit(1);
     });
+    
+    println!("{}", content_hash.hex());
 }
 
 fn handle_get(args: &GetArgs) {