usage/ctxsz: separate billed tokens from context size

- Rename Core.Usage() to Core.CtxSz() (estimated context size)
- New Core.Usage() returns billed token counts (in, out, requests)
- 9P: add ctxsz file, usage file now returns billed tokens
- /context command now shows context size summary
- Emacs mode-line reads ctxsz instead of usage

pkg/agent/core.go

@@ -429,7 +429,7 @@ func (s *agentCore) IsRunning() bool {
 	return s.currentAction.Load() != nil
 }
 
-func (s *agentCore) Usage() string {
+func (s *agentCore) CtxSz() string {
 	if s.session == nil {
 		return "no active session"
 	}
@@ -442,6 +442,14 @@ func (s *agentCore) Usage() string {
 	return fmt.Sprintf("%d / %d (%d%%)", estimated, ctxLen, pct)
 }
 
+func (s *agentCore) Usage() string {
+	if s.session == nil {
+		return "no active session"
+	}
+	return fmt.Sprintf("%d in, %d out, %d requests",
+		s.session.TotalInputTokens, s.session.TotalOutputTokens, s.session.TotalRequests)
+}
+
 func (s *agentCore) ListModels() string {
 	models := s.loopcfg.Backend.Models(context.Background())
 	return strings.Join(models, "\n")
@@ -804,6 +812,13 @@ func (s *agentCore) handleCommand(ctx context.Context, input string, handler Eve
 			handler(infoEvent("no active session"))
 			return true
 		}
+		ctxLen := s.loopcfg.Backend.ContextLength(ctx)
+		if ctxLen <= 0 {
+			ctxLen = defaultContextLength
+		}
+		estimated := s.session.estimateTokens()
+		pct := estimated * 100 / ctxLen
+		handler(infoEvent(fmt.Sprintf("~%d / %d tokens (%d%%)", estimated, ctxLen, pct)))
 		handler(infoEvent(strings.TrimRight(s.session.contextDebug(), "\n")))
 		return true
 
@@ -943,7 +958,7 @@ func (s *agentCore) handleCommand(ctx context.Context, input string, handler Eve
 			"  /cwd [path]      - show or change working directory",
 			"  /queued [pop|clear] - manage queued prompts",
 			"  /compact         - summarize conversation and compact context",
-			"  /context         - show context window debug info",
+			"  /context         - show context size and message breakdown",
 			"  /usage           - show token usage and context percentage",
 			"  /history         - dump bounded message history",
 			"  /clear           - clear session",

pkg/agent/types.go

@@ -67,7 +67,10 @@ type Core interface {
 	// ModelName returns the name of the active model.
 	ModelName() string
 
-	// Usage returns a one-line usage summary string.
+	// CtxSz returns the estimated context size as a one-line summary.
+	CtxSz() string
+
+	// Usage returns billed token counts as a one-line summary.
 	Usage() string
 
 	// ListModels returns available model names, one per line.