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/metrics/system_info.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 	agentPkg "github.com/yusing/godoxy/agent/pkg/agent"
+	"github.com/yusing/godoxy/internal/agentpool"
 	"github.com/yusing/godoxy/internal/metrics/period"
 	"github.com/yusing/godoxy/internal/metrics/systeminfo"
 	apitypes "github.com/yusing/goutils/apitypes"
@@ -49,9 +50,9 @@ func SystemInfo(c *gin.Context) {
 	}
 	c.Request.URL.RawQuery = query.Encode()
 
-	agent, ok := agentPkg.GetAgent(agentAddr)
+	agent, ok := agentpool.Get(agentAddr)
 	if !ok {
-		agent, ok = agentPkg.GetAgentByName(agentName)
+		agent, ok = agentpool.GetAgent(agentName)
 	}
 	if !ok {
 		c.JSON(http.StatusNotFound, apitypes.Error("agent_addr or agent_name not found"))