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/datetime/plugin.go

@@ -0,0 +1,23 @@
+package datetime
+
+import (
+	"context"
+	"time"
+
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/plugin"
+)
+
+type Plugin struct{}
+func New() *Plugin { return &Plugin{} }
+func (p *Plugin) Name() string { return "datetime" }
+func (p *Plugin) Version() string { return "1.0.0" }
+func (p *Plugin) Description() string { return "日期时间计算" }
+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:"datetime", Description:"获取当前日期时间", Parameters:map[string]any{}}}
+}
+func (p *Plugin) Execute(_ context.Context, _ string, _ map[string]any) (string, error) {
+	return time.Now().Format("2006-01-02 15:04:05"), nil
+}