maven: dialogue state scaffold (task 6)

- New internal/dialogue/ package: Session, SessionStore, InheritSlots
- Session with Intent, Slots, Timestamp, TTL
- SessionStore: in-memory map with TTL expiry, thread-safe
- InheritSlots: carries forward slots from previous turn
- Tests: expiry, store put/get/expiry, default TTL, slot inheritance

Co-Authored-By: opencode <opencode@anthropic.com>

internal/dialogue/session.go

@@ -0,0 +1,109 @@
+package dialogue
+
+import (
+	"sync"
+	"time"
+)
+
+type Intent string
+
+const (
+	IntentAct      Intent = "act"
+	IntentReminder Intent = "reminder"
+	IntentFact     Intent = "fact"
+	IntentNote     Intent = "note"
+	IntentQuery    Intent = "query"
+	IntentSystem   Intent = "system"
+)
+
+type Slots struct {
+	Time    time.Time
+	HasTime bool
+	Key     string
+	HasKey  bool
+	Text    string
+	Fn      string
+	Args    []string
+	HasFn   bool
+}
+
+type Session struct {
+	Intent    Intent
+	Slots     Slots
+	Timestamp time.Time
+	TTL       time.Duration
+}
+
+func (s *Session) IsExpired(now time.Time) bool {
+	return now.After(s.Timestamp.Add(s.TTL))
+}
+
+type SessionStore struct {
+	mu         sync.RWMutex
+	sessions   map[string]*Session
+	defaultTTL time.Duration
+}
+
+func NewSessionStore(defaultTTL time.Duration) *SessionStore {
+	if defaultTTL <= 0 {
+		defaultTTL = 2 * time.Minute
+	}
+	return &SessionStore{
+		sessions:   make(map[string]*Session),
+		defaultTTL: defaultTTL,
+	}
+}
+
+func (s *SessionStore) Get(id string, now time.Time) *Session {
+	s.mu.RLock()
+	sess, ok := s.sessions[id]
+	s.mu.RUnlock()
+	if !ok {
+		return nil
+	}
+	if sess.IsExpired(now) {
+		s.Delete(id)
+		return nil
+	}
+	return sess
+}
+
+func (s *SessionStore) Put(id string, sess *Session) {
+	if sess.TTL <= 0 {
+		sess.TTL = s.defaultTTL
+	}
+	s.mu.Lock()
+	s.sessions[id] = sess
+	s.mu.Unlock()
+}
+
+func (s *SessionStore) Delete(id string) {
+	s.mu.Lock()
+	delete(s.sessions, id)
+	s.mu.Unlock()
+}
+
+func InheritSlots(prev, cur Slots) Slots {
+	out := cur
+	if !out.HasTime && prev.HasTime {
+		out.Time = prev.Time
+		out.HasTime = true
+	}
+	if !out.HasKey && prev.HasKey {
+		out.Key = prev.Key
+		out.HasKey = true
+	}
+	if out.Text == "" && prev.Text != "" {
+		out.Text = prev.Text
+	}
+	if !out.HasFn && prev.HasFn {
+		out.Fn = prev.Fn
+		out.HasFn = true
+	}
+	if len(out.Args) == 0 && len(prev.Args) > 0 {
+		out.Args = append([]string(nil), prev.Args...)
+	}
+	return out
+}
+
+