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/worker/client.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"log"
 	"net"
 	"sync"
 	"time"
@@ -95,9 +96,13 @@ func (c *Client) call(ctx context.Context, m Method, params any, out any) error
 	// half-sent frame would desync the stream. Teardown is the clean
 	// recovery; the next call re-dials.
 	if dl, ok := ctx.Deadline(); ok {
-		_ = c.c.SetDeadline(dl)
+		if err := c.c.SetDeadline(dl); err != nil {
+			log.Printf("worker: set deadline: %v", err)
+		}
 	} else {
-		_ = c.c.SetDeadline(time.Now().Add(defaultCallTimeout))
+		if err := c.c.SetDeadline(time.Now().Add(defaultCallTimeout)); err != nil {
+			log.Printf("worker: set deadline: %v", err)
+		}
 	}
 	defer c.c.SetDeadline(time.Time{})