Sort session list by creation time (newest first)

Backend changes:
- Add CreatedAt() to Session interface
- Implement CreatedAt() in tmux.Session (field already existed)
- Sort Manager.List() by creation time descending

Benefits:
- Deterministic ordering (no more random map iteration)
- Newest sessions appear first (most useful)
- All clients benefit immediately
- head -1 now reliably gets most recent session

README:
- Update example to use head -1 for newest session
- Document that list is sorted newest first

README.md

@@ -186,12 +186,12 @@ echo '{"to":"a3f2b9d1","type":"REVIEW_REQUEST","subject":"...","body":"..."}' |
 # Create session
 echo 'new claude /home/user/project' | 9p write agent/ctl
 
-# List sessions (order is non-deterministic)
+# List sessions (newest first)
 9p read agent/list
 # Output: <id> <backend> <state> <alias> <cwd>
 
-# Use a specific session ID
-ID="a1b2c3d4"  # Replace with actual ID from list
+# Get most recent session ID
+ID=$(9p read agent/list | head -1 | awk '{print $1}')
 
 # Send prompt via mailbox
 echo '{"to":"'$ID'","type":"PROMPT_REQUEST","subject":"User prompt","body":"Hello"}' | 9p write user/mail

internal/backend/backend.go

@@ -78,6 +78,9 @@ type Session interface {
 	
 	// Tasks returns the session tasks (empty if not specified)
 	Tasks() []string
+	
+	// CreatedAt returns when the session was created
+	CreatedAt() time.Time
 }
 
 // SessionMetadata provides information about a session

internal/backend/tmux/session.go

@@ -193,6 +193,13 @@ func (s *Session) Tasks() []string {
 	return s.tasks
 }
 
+// CreatedAt returns when the session was created
+func (s *Session) CreatedAt() time.Time {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.createdAt
+}
+
 // GetPid gets the process ID
 func (s *Session) GetPid() int {
 	s.mu.Lock()

internal/session/manager.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"sort"
 	"sync"
 	"time"
 )
@@ -133,14 +134,28 @@ func (m *Manager) GetAlias(id string) string {
 	return sess.Metadata().Alias
 }
 
-// List returns all session IDs
+// List returns all session IDs, sorted by creation time (newest first)
 func (m *Manager) List() []string {
 	m.mu.RLock()
 	defer m.mu.RUnlock()
 
-	ids := make([]string, 0, len(m.sessions))
-	for id := range m.sessions {
-		ids = append(ids, id)
+	type entry struct {
+		id      string
+		created time.Time
+	}
+
+	entries := make([]entry, 0, len(m.sessions))
+	for id, sess := range m.sessions {
+		entries = append(entries, entry{id, sess.CreatedAt()})
+	}
+
+	sort.Slice(entries, func(i, j int) bool {
+		return entries[i].created.After(entries[j].created) // newest first
+	})
+
+	ids := make([]string, len(entries))
+	for i, e := range entries {
+		ids[i] = e.id
 	}
 	return ids
 }