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/tool/tool.go

@@ -24,6 +24,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"log"
 	"os/exec"
 	"strings"
 	"time"
@@ -117,6 +118,7 @@ func NewMatcher(api API) *Matcher { return &Matcher{api: api} }
 func (m *Matcher) names() []string {
 	ts, err := m.api.ListTools(context.Background(), "enabled")
 	if err != nil {
+		log.Printf("tool: list enabled tools: %v", err)
 		return nil
 	}
 	names := make([]string, len(ts))