piper: pipe cleanup on early return, write error; misc error hygiene

piper_handler: close stdin/stdout pipes on Start() failure and on
WriteString error instead of leaking fds. Propagate WriteString error.

worker/client: log SetDeadline errors instead of discarding them.

voice/session: pushAudio marshals params inline and returns the marshal
error instead of swallowing it via mustParams (removed).

tool/matcher: log ListTools errors instead of silently returning an
empty allowlist that refuses every act.

config: applyDefaults now sets RouterThreshold and ToolTimeout defaults
so consumers self-contained defaults are belt-and-suspenders.

internal/voice/session.go

@@ -69,7 +69,11 @@ func (s *Session) pushAudio(p AudioNudgePush) error {
 	if s.closed || s.conn == nil {
 		return fmt.Errorf("voice: session %d closed: %w", s.ID, ErrNoSession)
 	}
-	return writeFrame(s.conn, Push{Kind: PushKindAudioNudge, Params: mustParams(p)})
+	params, err := json.Marshal(p)
+	if err != nil {
+		return fmt.Errorf("voice: marshal push params: %w", err)
+	}
+	return writeFrame(s.conn, Push{Kind: PushKindAudioNudge, Params: params})
 }
 
 func (s *Session) shutdown() {
@@ -167,11 +171,3 @@ func (r *Sessions) PushToMostRecent(ctx context.Context, p AudioNudgePush) error
 	return best.pushAudio(p)
 }
 
-// helper that returns a fixed nil-error marshal so the push call site is short.
-func mustParams(v any) json.RawMessage {
-	if v == nil {
-		return nil
-	}
-	b, _ := jsonMarshal(v)
-	return b
-}