fix(socketio): remove broken client-initiated ping (engine.io v4)

The socket.io client was sending its own EIO ping frames every
pingInterval (default 25s). That's wrong for engine.io v4: in v4 the
SERVER initiates pings and the CLIENT must respond with pong inside
pingTimeout, else the server closes the connection. Client-initiated
pings get misinterpreted by Fabric's NestJS socket.io backend, which
quietly closes the connection — producing the warn-flap every ~25s:

  inbound: socket ended; reconnecting
    err="read: failed to get reader: received close frame:
         status = StatusNoStatusRcvd and reason = \"\""

Fix:
- delete pingLoop() entirely
- delete the pingPeriod/pingTimeout struct fields + their assignments
  in recvOpen (server enforces both anyway; client doesn't need them)
- keep the eioPing case in handlePacket (already correct — responds
  with pong)
- drop the now-unused "time" import

End-to-end verified on live Fabric:
- Restarted Plexum at 20:17:35; watched for 90+ seconds
- ZERO "socket ended" events (vs. ~3-4 per 90s before the fix)
- Channel inbound still delivers: alice posted seq=20 → gem agent
  (gemini CLI) replied seq=21 "pong"

The plugin no longer flaps. Reconnect backoff machinery (1s→60s)
stays in place as a safety net for genuine network drops.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/socketio/client.go

@@ -24,7 +24,6 @@ import (
 	"fmt"
 	"net/url"
 	"sync"
-	"time"
 
 	"nhooyr.io/websocket"
 )
@@ -65,11 +64,9 @@ type Client struct {
 
 	// Read-only after Connect; mutating during a live connection is
 	// undefined.
-	handlers   map[string]Handler
-	handlerMu  sync.RWMutex
-	conn       *websocket.Conn
-	pingPeriod time.Duration // from server "open" packet
-	pingTimeout time.Duration
+	handlers  map[string]Handler
+	handlerMu sync.RWMutex
+	conn      *websocket.Conn
 
 	// closed-on-disconnect; Connect returns when this fires.
 	disconnected chan struct{}
@@ -168,10 +165,12 @@ func (c *Client) Connect(ctx context.Context) error {
 		return fmt.Errorf("CONNECT: %w", err)
 	}
 
-	pingCtx, cancelPing := context.WithCancel(ctx)
-	defer cancelPing()
-	go c.pingLoop(pingCtx)
-
+	// Engine.IO v4 keepalive contract: the SERVER pings every
+	// pingInterval; we MUST pong within pingTimeout or it disconnects.
+	// We do NOT initiate pings ourselves (an earlier version did, which
+	// caused Fabric guild backend to close the socket every ~25s — the
+	// client-side ping was misinterpreted as a misbehaving peer). Server
+	// pings are answered by handlePacket's eioPing case.
 	for {
 		_, data, err := conn.Read(ctx)
 		if err != nil {
@@ -197,7 +196,10 @@ func (c *Client) Disconnect() {
 	}
 }
 
-// recvOpen reads the EIO "open" frame and stashes ping intervals.
+// recvOpen reads the EIO "open" frame. The server announces its
+// pingInterval + pingTimeout there; we just acknowledge — the
+// keepalive cadence is server-driven (we pong on demand, see
+// handlePacket's eioPing case).
 func (c *Client) recvOpen(ctx context.Context) error {
 	_, data, err := c.conn.Read(ctx)
 	if err != nil {
@@ -206,19 +208,14 @@ func (c *Client) recvOpen(ctx context.Context) error {
 	if len(data) < 2 || data[0] != eioOpen {
 		return fmt.Errorf("expected EIO open, got %q", string(data))
 	}
+	// Parse just to fail fast on malformed open; the values are
+	// informational (server enforces them).
 	var info struct {
-		Sid          string `json:"sid"`
-		PingInterval int    `json:"pingInterval"`
-		PingTimeout  int    `json:"pingTimeout"`
+		Sid string `json:"sid"`
 	}
 	if err := json.Unmarshal(data[1:], &info); err != nil {
 		return fmt.Errorf("parse open: %w", err)
 	}
-	c.pingPeriod = time.Duration(info.PingInterval) * time.Millisecond
-	c.pingTimeout = time.Duration(info.PingTimeout) * time.Millisecond
-	if c.pingPeriod <= 0 {
-		c.pingPeriod = 25 * time.Second // EIO default
-	}
 	return nil
 }
 
@@ -238,35 +235,16 @@ func (c *Client) sendConnect(ctx context.Context, auth map[string]any) error {
 	return c.conn.Write(ctx, websocket.MessageText, body)
 }
 
-// pingLoop sends EIO ping frames per server-supplied interval.
-func (c *Client) pingLoop(ctx context.Context) {
-	ticker := time.NewTicker(c.pingPeriod)
-	defer ticker.Stop()
-	for {
-		select {
-		case <-ctx.Done():
-			return
-		case <-ticker.C:
-			if c.conn == nil {
-				return
-			}
-			writeCtx, cancel := context.WithTimeout(ctx, c.pingTimeout)
-			err := c.conn.Write(writeCtx, websocket.MessageText, []byte{eioPing})
-			cancel()
-			if err != nil {
-				return
-			}
-		}
-	}
-}
-
 // handlePacket inspects the first byte (EIO type) + dispatches.
 func (c *Client) handlePacket(data []byte) error {
 	switch data[0] {
 	case eioPong:
-		return nil // server responding to our ping (or vice versa)
+		// Server doesn't normally send unsolicited pongs in v4; ignore
+		// if it does. (Future: could be used to confirm an upgrade ack.)
+		return nil
 	case eioPing:
-		// Server-initiated ping; reply with pong.
+		// Server keepalive ping — MUST respond with pong inside
+		// server's pingTimeout, else server closes the connection.
 		if c.conn == nil {
 			return nil
 		}