[fix](trx-rs): install binaries under user home

Default installation and removal to /home/sjg/.local/bin while preserving explicit prefix and binary-directory overrides. Update the service examples and README to match.

Assisted-By: OpenAI Codex (GPT-5)
Signed-off-by: Stan Grams <sjg@haxx.space>

README.md

@@ -126,12 +126,12 @@ script/install.sh
 ```
 
 This builds in release mode, installs `trx-server`, `trx-client`, and
-`trx-configurator` to `/usr/local/bin` (using `sudo` only if needed), seeds
+`trx-configurator` to `~/.local/bin`, seeds
 `~/.config/trx-rs/trx-rs.toml` from the example (existing config is never
 overwritten), and installs `~/.config/systemd/user/{trx-server,trx-client}.service`.
 
 ```bash
-script/install.sh --prefix ~/.local   # install to ~/.local/bin instead
+script/install.sh --prefix /opt/trx-rs # choose another installation prefix
 script/install.sh --no-sdr            # build without SoapySDR support
 script/install.sh --enable-now        # also enable + start the services now
 script/install.sh --help              # all options