feat: 人格文档双层架构 — persona.md全量注入system prompt + RAG黑名单过滤

- PersonaConfig 新增 NarrativePersona 字段，在 system prompt 的「你的身份」和「你的性格」之间插入「## 你的叙事人格」
- Loader.NewLoader 自动读取同目录 persona.md 注入所有 config；Reload 同步热更新
- RAG IngestDirectory 跳过 persona.md 和 _index.md，避免重复检索浪费token
- 新增 docs/dev-plan/12-next-phase-optimization.md 下一阶段优化大纲

Co-Authored-By: Claude <noreply@anthropic.com>

backend/ai-core/internal/persona/loader.go

@@ -3,6 +3,7 @@ package persona
 import (
 	"fmt"
 	"os"
+	"path/filepath"
 	"sync"
 
 	"gopkg.in/yaml.v3"
@@ -54,6 +55,17 @@ func NewLoader(personaDir string) (*Loader, error) {
 		return nil, fmt.Errorf("未找到任何人格配置文件")
 	}
 
+	// Load narrative persona from persona.md (same directory as YAML configs).
+	// This is the full-text personality narrative injected verbatim into the system prompt.
+	personaMDPath := filepath.Join(personaDir, "persona.md")
+	if data, err := os.ReadFile(personaMDPath); err == nil {
+		// Attach to every loaded persona config.
+		narrative := string(data)
+		for _, cfg := range l.configs {
+			cfg.NarrativePersona = narrative
+		}
+	}
+
 	return l, nil
 }
 
@@ -82,6 +94,12 @@ func (l *Loader) Reload(name string, path string) error {
 	}
 
 	l.mu.Lock()
+	// Re-read persona.md to keep narrative persona in sync.
+	personaDir := filepath.Dir(path)
+	personaMDPath := filepath.Join(personaDir, "persona.md")
+	if data, err := os.ReadFile(personaMDPath); err == nil {
+		cfg.NarrativePersona = string(data)
+	}
 	l.configs[name] = &cfg
 	l.mu.Unlock()