[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>

README.md

@@ -161,16 +161,6 @@ Audio is transported as Opus between server, client, and browser.
 - `trx-client` relays audio to the HTTP frontend
 - Browsers connect over `/audio`
 
-## Plugins
-
-Both binaries can discover shared-library plugins through:
-
-- `./plugins`
-- `~/.config/trx-rs/plugins`
-- `TRX_PLUGIN_DIRS`
-
-See [`examples/trx-plugin-example/README.md`](examples/trx-plugin-example/README.md).
-
 ## Documentation
 
 - [User Manual](docs/User-Manual.md): configuration, features, and usage