feat(plugins): add 10 tool plugins and wire full service stack in main.go

- plugins: calculator, datetime, crypto, random, text, json, markdown
- plugins: web-search (SearXNG stub), web-fetch, file, http (stubs)
- main.go: full wiring — config→db→nats→plugins→voice→memory→
  persona→llm→context→subsession→orchestrator→gateway→ws
- 15 packages passing, 11 plugins loaded at startup

backend/plugins/web-search/plugin.go

@@ -0,0 +1,19 @@
+package websearch
+import (
+    "context"
+    "git.yeij.top/AskaEth/Cyrene_v2/internal/plugin"
+)
+type Plugin struct{ searxngURL string }
+func New(url string) *Plugin { return &Plugin{searxngURL: url} }
+func (p *Plugin) Name() string { return "web-search" }
+func (p *Plugin) Version() string { return "1.0.0" }
+func (p *Plugin) Description() string { return "网络搜索" }
+func (p *Plugin) Init(_ context.Context, _ plugin.PluginConfig) error { return nil }
+func (p *Plugin) Close() error { return nil }
+func (p *Plugin) Enabled() bool { return p.searxngURL != "" }
+func (p *Plugin) Tools() []plugin.ToolDefinition {
+    return []plugin.ToolDefinition{{Name:"web_search", Description:"互联网搜索", Parameters:map[string]any{"query": map[string]string{"type":"string"}}}}
+}
+func (p *Plugin) Execute(_ context.Context, _ string, _ map[string]any) (string, error) {
+    return "[搜索] 引擎未配置", nil
+}