feat(plugins): implement web-search, web-fetch, http and file tools

- web-search: real SearXNG /search?format=json query, top-5 formatted
  results (was a 'not configured' stub)
- web-fetch: fetch page, strip scripts/styles/tags, truncate to 4000
  runes for the LLM context
- http: generic request tool (GET/POST/PUT/DELETE/PATCH) with scheme
  and method validation, status + truncated body
- file: sandboxed read/write/list inside the data directory with path
  escape rejection; split into file_read/file_write/file_list
- all tool schemas converted to proper JSON Schema; httptest/tempdir
  tests for each plugin

backend/plugins/web-fetch/plugin.go

@@ -1,19 +1,96 @@
 package webfetch
+
 import (
-    "context"
-    "git.yeij.top/AskaEth/Cyrene_v2/internal/plugin"
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"regexp"
+	"strings"
+	"time"
+
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/plugin"
 )
-type Plugin struct{}
-func New() *Plugin { return &Plugin{} }
-func (p *Plugin) Name() string { return "web-fetch" }
-func (p *Plugin) Version() string { return "1.0.0" }
+
+type Plugin struct{ client *http.Client }
+
+func New() *Plugin { return &Plugin{client: &http.Client{Timeout: 20 * time.Second}} }
+
+func (p *Plugin) Name() string        { return "web-fetch" }
+func (p *Plugin) Version() string     { return "1.0.0" }
 func (p *Plugin) Description() string { return "网页抓取" }
 func (p *Plugin) Init(_ context.Context, _ plugin.PluginConfig) error { return nil }
-func (p *Plugin) Close() error { return nil }
-func (p *Plugin) Enabled() bool { return true }
+func (p *Plugin) Close() error        { return nil }
+func (p *Plugin) Enabled() bool       { return true }
+
 func (p *Plugin) Tools() []plugin.ToolDefinition {
-    return []plugin.ToolDefinition{{Name:"web_fetch", Description:"获取网页内容", Parameters:map[string]any{"url": map[string]string{"type":"string"}}}}
+	return []plugin.ToolDefinition{{
+		Name:        "web_fetch",
+		Description: "Fetch a web page and return its text content (HTML stripped, truncated).",
+		Parameters: map[string]any{
+			"type": "object",
+			"properties": map[string]any{
+				"url": map[string]string{"type": "string", "description": "Page URL (http/https)"},
+			},
+			"required": []string{"url"},
+		},
+	}}
 }
-func (p *Plugin) Execute(_ context.Context, _ string, _ map[string]any) (string, error) {
-    return "[抓取] 功能占位", nil
+
+var (
+	reScript   = regexp.MustCompile(`(?is)<(script|style|noscript)[^>]*>.*?</(script|style|noscript)>`)
+	reTags     = regexp.MustCompile(`(?s)<[^>]+>`)
+	reBlank    = regexp.MustCompile(`\n\s*\n+`)
+	maxBody    = 1 << 20   // 1MB
+	maxContent = 4000      // runes returned to the LLM
+)
+
+func (p *Plugin) Execute(ctx context.Context, _ string, params map[string]any) (string, error) {
+	rawURL, _ := params["url"].(string)
+	if !strings.HasPrefix(rawURL, "http://") && !strings.HasPrefix(rawURL, "https://") {
+		return "", fmt.Errorf("url must start with http:// or https://")
+	}
+
+	req, err := http.NewRequestWithContext(ctx, "GET", rawURL, nil)
+	if err != nil {
+		return "", err
+	}
+	req.Header.Set("User-Agent", "Mozilla/5.0 (compatible; Cyrene/2.0)")
+
+	resp, err := p.client.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("fetch: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		return "", fmt.Errorf("fetch API %d", resp.StatusCode)
+	}
+
+	body, err := io.ReadAll(io.LimitReader(resp.Body, int64(maxBody)))
+	if err != nil {
+		return "", fmt.Errorf("read body: %w", err)
+	}
+
+	text := htmlToText(string(body))
+	if runes := []rune(text); len(runes) > maxContent {
+		text = string(runes[:maxContent]) + "\n…（内容过长已截断）"
+	}
+	if text == "" {
+		return "页面无可用文本内容", nil
+	}
+	return text, nil
+}
+
+// htmlToText strips scripts/styles/tags and collapses whitespace.
+func htmlToText(html string) string {
+	html = reScript.ReplaceAllString(html, " ")
+	html = reTags.ReplaceAllString(html, "\n")
+	html = reBlank.ReplaceAllString(html, "\n")
+	var lines []string
+	for _, line := range strings.Split(html, "\n") {
+		if s := strings.TrimSpace(line); s != "" {
+			lines = append(lines, s)
+		}
+	}
+	return strings.Join(lines, "\n")
 }