CLI: async runtime + proper RPC fallback plumbing

- main.rs: use #[tokio::main] so CLI has a runtime available
- memory.rs: make run_with_local_store async (no more runtime creation)
- mcp_server.rs: cache socket connection in OnceLock, use block_in_place
  for async fallback when socket unavailable

Fixes "cannot start a runtime from within a runtime" panic when CLI
falls back to local store.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/main.rs

@@ -476,7 +476,8 @@ impl Run for AdminCmd {
     }
 }
 
-fn main() {
+#[tokio::main]
+async fn main() {
     std::panic::set_backtrace_style(std::panic::BacktraceStyle::Short);
 
     // Handle --help ourselves for expanded subcommand display