agent: debounce session persistence

saveSession() now sets a dirty flag and starts a 2-second timer
instead of immediately serializing and writing the full message
history. For a 50-step turn, this reduces 50 JSON marshals + disk
writes down to ~25 (one every 2s) plus a guaranteed flush at
turn end and on close.

flushSave() is called:
- At end of turn (after setState idle)
- On interrupt
- On Close()
- When the 2s timer fires (if still dirty)

pkg/agent/core.go

@@ -289,6 +289,11 @@ type agent struct {
 	submitMu        sync.Mutex // serializes Submit calls (commands + turns)
 	warnedContext   bool       // true after a context-usage warning; cleared on compaction
 	auditLog        *olog.Logger
+
+	// Debounced session persistence
+	saveMu      sync.Mutex
+	saveDirty   bool
+	saveTimer   *time.Timer
 }
 
 // ToolCallCount returns the total number of tool calls executed in this
@@ -460,6 +465,7 @@ func NewAgentCore(cfg AgentCoreConfig) Core {
 // Close releases resources for this session, including its tmpdir.
 func (s *agent) Close() {
 	s.log.Debug("Close() session=%q", s.sessionID)
+	s.flushSave()
 	if s.runtime != nil && s.runtime.Dispatcher != nil {
 		if srv, ok := s.runtime.Dispatcher.GetServer("execute"); ok {
 			if c, ok := srv.(interface{ Close() }); ok {
@@ -805,6 +811,27 @@ func (s *agent) runCompact(ctx context.Context, trigger string) (int, error) {
 }
 
 func (s *agent) saveSession() {
+	s.saveMu.Lock()
+	s.saveDirty = true
+	if s.saveTimer == nil {
+		s.saveTimer = time.AfterFunc(2*time.Second, s.flushSave)
+	}
+	s.saveMu.Unlock()
+}
+
+// flushSave immediately persists the session if dirty.
+func (s *agent) flushSave() {
+	s.saveMu.Lock()
+	dirty := s.saveDirty
+	s.saveDirty = false
+	if s.saveTimer != nil {
+		s.saveTimer.Stop()
+		s.saveTimer = nil
+	}
+	s.saveMu.Unlock()
+	if !dirty {
+		return
+	}
 	if s.session == nil || s.sessionID == "" || s.sessionsDir == "" {
 		return
 	}
@@ -1292,6 +1319,7 @@ func (s *agent) executeTurn(ctx context.Context, input string) string {
 	s.mu.Unlock()
 	replyBuf.Reset()
 	s.setState("idle")
+	s.flushSave()
 
 	if err != nil {
 		// Keep completed work — only remove cancelled tool results.