feat(ai): session harness contract — incomplete, run_id, confirm bind

Add docs/16 for session state machine, incomplete/resume, queue semantics,
and agent-loop/harness/MCP gap review. Wire RunOutcome on max-rounds,
run_id through audit, TrimMessages/Redact hooks, ConfirmBindHash on write
confirm path; session file skeleton without HTTP yet. No push.

server/internal/ai/session.go

@@ -0,0 +1,89 @@
+package ai
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// Session 桌面本地会话（目标态落盘；HTTP 路由可后挂）。
+type Session struct {
+	ID        string         `json:"id"`
+	Title     string         `json:"title,omitempty"`
+	Status    string         `json:"status"` // idle|running|incomplete|completed|failed|cancelled
+	CreatedAt string         `json:"createdAt"`
+	UpdatedAt string         `json:"updatedAt"`
+	Messages  []Message      `json:"messages"`
+	Runs      []SessionRun   `json:"runs,omitempty"`
+}
+
+// SessionRun 一次运行摘要。
+type SessionRun struct {
+	RunID             string `json:"runId"`
+	Outcome           string `json:"outcome"`
+	IncompleteReason  string `json:"incompleteReason,omitempty"`
+	Resumable         bool   `json:"resumable"`
+	StepsUsed         int    `json:"stepsUsed"`
+	StepsBudget       int    `json:"stepsBudget"`
+	FinishedAt        string `json:"finishedAt"`
+}
+
+func sessionsDir() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, ".opsxdb", "ai-sessions"), nil
+}
+
+// SessionPath 返回会话文件路径。
+func SessionPath(id string) (string, error) {
+	dir, err := sessionsDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, id+".json"), nil
+}
+
+// SaveSession 脱敏后写入磁盘。
+func SaveSession(s Session) error {
+	if s.ID == "" {
+		return fmt.Errorf("session id required")
+	}
+	s.Messages = RedactMessages(s.Messages)
+	s.UpdatedAt = time.Now().Format(time.RFC3339)
+	if s.CreatedAt == "" {
+		s.CreatedAt = s.UpdatedAt
+	}
+	path, err := SessionPath(s.ID)
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0700); err != nil {
+		return err
+	}
+	b, err := json.MarshalIndent(s, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(path, b, 0600)
+}
+
+// LoadSession 读会话；不存在返回 os.ErrNotExist。
+func LoadSession(id string) (Session, error) {
+	path, err := SessionPath(id)
+	if err != nil {
+		return Session{}, err
+	}
+	b, err := os.ReadFile(path)
+	if err != nil {
+		return Session{}, err
+	}
+	var s Session
+	if err := json.Unmarshal(b, &s); err != nil {
+		return Session{}, err
+	}
+	return s, nil
+}