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/json/plugin.go

@@ -0,0 +1,23 @@
+package json
+import (
+    "context"; encjson "encoding/json"
+    "git.yeij.top/AskaEth/Cyrene_v2/internal/plugin"
+)
+type Plugin struct{}
+func New() *Plugin { return &Plugin{} }
+func (p *Plugin) Name() string { return "json" }
+func (p *Plugin) Version() string { return "1.0.0" }
+func (p *Plugin) Description() string { return "JSON处理" }
+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) Tools() []plugin.ToolDefinition {
+    return []plugin.ToolDefinition{{Name:"json", Description:"JSON解析/格式化", Parameters:map[string]any{"json_text": map[string]string{"type":"string"}, "action":map[string]string{"type":"string"}}}}
+}
+func (p *Plugin) Execute(_ context.Context, _ string, params map[string]any) (string, error) {
+    t,_ := params["json_text"].(string)
+    var v any
+    if err := encjson.Unmarshal([]byte(t), &v); err != nil { return "", err }
+    b,_ := encjson.MarshalIndent(v, "", "  ")
+    return string(b), nil
+}