[refactor](trx-rs): remove shared-library plugin system

Drop the plugin loading infrastructure (libloading-based dynamic .so/.dylib/.dll
loading) from both trx-server and trx-client. The feature was unused and posed an
unnecessary security risk by executing arbitrary native code from disk.

Removed:
- src/trx-app/src/plugins.rs (plugin discovery, validation, FFI registration)
- examples/trx-plugin-example/ (cdylib example plugin)
- libloading dependency from trx-app
- load_backend_plugins / load_frontend_plugins calls from server and client
- Plugin documentation from README.md and CLAUDE.md

https://claude.ai/code/session_01DTEUpz3XPUeWmz74NeaFgb
Signed-off-by: Claude <noreply@anthropic.com>

src/trx-server/src/main.rs

@@ -14,7 +14,6 @@ use std::collections::HashMap;
 use std::collections::HashSet;
 use std::net::{IpAddr, SocketAddr};
 use std::path::PathBuf;
-use std::ptr::NonNull;
 use std::sync::Arc;
 use std::time::Duration;
 
@@ -27,7 +26,7 @@ use tracing::{error, info, warn};
 
 use trx_core::audio::AudioStreamInfo;
 
-use trx_app::{init_logging, load_backend_plugins, normalize_name};
+use trx_app::{init_logging, normalize_name};
 use trx_backend::{register_builtin_backends_on, RegistrationContext, RigAccess};
 use trx_core::rig::controller::{AdaptivePolling, ExponentialBackoff};
 use trx_core::rig::request::RigRequest;
@@ -854,9 +853,6 @@ async fn main() -> DynResult<()> {
 
     init_logging(cfg.general.log_level.as_deref());
 
-    let bootstrap_ctx_ptr = NonNull::from(&mut bootstrap_ctx).cast();
-    let _plugin_libs = load_backend_plugins(bootstrap_ctx_ptr);
-
     if let Some(ref path) = config_path {
         info!("Loaded configuration from {}", path.display());
     }