session: add plan, prevPrompt, peers to Session

Extend the core session with fields that are genuinely session
concerns currently duplicated in 9p/session:
- Plan (markdown checklist, survives compaction)
- PrevPrompt (last submitted prompt text)
- Peers (bidirectional session links)

The 9p/session layer can delegate to these in a future change.

session/session.go

@@ -26,6 +26,10 @@ type Session struct {
 	envMu sync.RWMutex
 	env   map[string]string
 
+	plan       []byte
+	prevPrompt string
+	peers      map[string]bool // peer session IDs (bidirectional)
+
 	changeMu   sync.Mutex
 	changeCond *sync.Cond
 }
@@ -156,3 +160,66 @@ func (s *Session) notifyChange() {
 	s.changeCond.Broadcast()
 	s.changeMu.Unlock()
 }
+
+// Plan returns the session plan.
+func (s *Session) Plan() []byte {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.plan
+}
+
+// SetPlan updates the session plan.
+func (s *Session) SetPlan(p []byte) {
+	s.mu.Lock()
+	s.plan = p
+	s.mu.Unlock()
+}
+
+// PrevPrompt returns the last submitted prompt.
+func (s *Session) PrevPrompt() string {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.prevPrompt
+}
+
+// SetPrevPrompt stores the last submitted prompt.
+func (s *Session) SetPrevPrompt(p string) {
+	s.mu.Lock()
+	s.prevPrompt = p
+	s.mu.Unlock()
+}
+
+// HasPeer reports whether peerID is linked to this session.
+func (s *Session) HasPeer(id string) bool {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.peers[id]
+}
+
+// AddPeer adds a bidirectional peer link.
+func (s *Session) AddPeer(id string) {
+	s.mu.Lock()
+	if s.peers == nil {
+		s.peers = make(map[string]bool)
+	}
+	s.peers[id] = true
+	s.mu.Unlock()
+}
+
+// RemovePeer removes a peer link.
+func (s *Session) RemovePeer(id string) {
+	s.mu.Lock()
+	delete(s.peers, id)
+	s.mu.Unlock()
+}
+
+// Peers returns a copy of all peer IDs.
+func (s *Session) Peers() []string {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	out := make([]string, 0, len(s.peers))
+	for id := range s.peers {
+		out = append(out, id)
+	}
+	return out
+}