refactor(agent): extract agent pool and HTTP utilities to dedicated package

Moved non-agent-specific logic from agent/pkg/agent/ to internal/agentpool/:
- pool.go: Agent pool management (Get, Add, Remove, List, Iter, etc.)
- http_requests.go: HTTP utilities (health checks, forwarding, websockets, reverse proxy)
- agent.go: Agent struct with HTTP client management

This separates general-purpose pool management from agent-specific configuration,
improving code organization and making the agent package focused on agent config only.

internal/api/v1/agent/list.go

@@ -5,7 +5,7 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
-	"github.com/yusing/godoxy/agent/pkg/agent"
+	"github.com/yusing/godoxy/internal/agentpool"
 	"github.com/yusing/goutils/http/httpheaders"
 	"github.com/yusing/goutils/http/websocket"
 
@@ -19,15 +19,15 @@ import (
 // @Tags			agent,websocket
 // @Accept			json
 // @Produce		json
-// @Success		200	{array}		Agent
+// @Success		200	{array}		agent.AgentConfig
 // @Failure		403	{object}	apitypes.ErrorResponse
 // @Router			/agent/list [get]
 func List(c *gin.Context) {
 	if httpheaders.IsWebsocket(c.Request.Header) {
 		websocket.PeriodicWrite(c, 10*time.Second, func() (any, error) {
-			return agent.ListAgents(), nil
+			return agentpool.List(), nil
 		})
 	} else {
-		c.JSON(http.StatusOK, agent.ListAgents())
+		c.JSON(http.StatusOK, agentpool.List())
 	}
 }