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/file/plugin_test.go

@@ -0,0 +1,57 @@
+package file
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/plugin"
+)
+
+func setup(t *testing.T) *Plugin {
+	t.Helper()
+	p := New()
+	if err := p.Init(context.Background(), plugin.PluginConfig{DataDir: t.TempDir()}); err != nil {
+		t.Fatalf("Init: %v", err)
+	}
+	return p
+}
+
+func TestWriteReadList(t *testing.T) {
+	p := setup(t)
+	ctx := context.Background()
+
+	out, err := p.Execute(ctx, "file_write", map[string]any{"path": "notes/todo.txt", "content": "买牛奶"})
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	if !strings.Contains(out, "已写入") {
+		t.Errorf("unexpected write output: %q", out)
+	}
+
+	out, err = p.Execute(ctx, "file_read", map[string]any{"path": "notes/todo.txt"})
+	if err != nil || out != "买牛奶" {
+		t.Errorf("read = %q, err = %v", out, err)
+	}
+
+	out, err = p.Execute(ctx, "file_list", map[string]any{"path": "notes"})
+	if err != nil || !strings.Contains(out, "todo.txt") {
+		t.Errorf("list = %q, err = %v", out, err)
+	}
+}
+
+func TestPathEscapeRejected(t *testing.T) {
+	p := setup(t)
+	for _, path := range []string{"../secret", "../../etc/passwd", "/etc/passwd"} {
+		if _, err := p.Execute(context.Background(), "file_read", map[string]any{"path": path}); err == nil {
+			t.Errorf("path %q should be rejected", path)
+		}
+	}
+}
+
+func TestReadMissingFile(t *testing.T) {
+	p := setup(t)
+	if _, err := p.Execute(context.Background(), "file_read", map[string]any{"path": "nope.txt"}); err == nil {
+		t.Error("expected error for missing file")
+	}
+}