[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>

CLAUDE.md

@@ -88,10 +88,6 @@ The rig controller (`src/trx-core/src/rig/controller/`) is the central state man
 
 Signal decoders run as background tasks in `trx-server`, consuming decoded audio. `trx-ftx` provides the FT8/FT4/FT2 decoder in pure Rust. Decoded frames can be forwarded to PSKReporter and APRS-IS (IGate) uplinks, or logged via `trx-decode-log`.
 
-### Plugin system
-
-Both `trx-server` and `trx-client` can load shared-library plugins exporting a `trx_register` symbol. Search paths: `./plugins`, `~/.config/trx-rs/plugins`, `TRX_PLUGIN_DIRS` env var.
-
 ## Commit Format
 
 ```