feat: implement isolated model worker with auto-restart and hot-reload

Introduce a supervised child process for capture, VAD, and recognition,
enabling automatic restart on crash/hang and hot-reload via SIGHUP.
Add `--model-auto-restart`, `--model-timeout`, and
`--model-shutdown-timeout` options. Refactor `Run()` with lifecycle
hooks for proper process group cleanup. Update README with new
behavior and CLI flags.

src/models/sherpa.go

@@ -25,6 +25,11 @@ type Resources struct {
 	vad        *sherpa.VoiceActivityDetector
 }
 
+func Validate(settings config.Settings) error {
+	_, err := validatePaths(settings.ModelsDir, settings.Mode)
+	return err
+}
+
 func New(settings config.Settings) (*Resources, error) {
 	paths, err := validatePaths(settings.ModelsDir, settings.Mode)
 	if err != nil {