add React() for emoji reactions on assistant messages

New method on the Core interface that appends a lightweight user
message to session history without triggering a model turn. The
agent sees the reaction as context on its next turn.

Used by the 9P react file: echo emoji > s/{id}/react

pkg/agent/core.go

@@ -607,6 +607,15 @@ func (s *agent) InjectSystemEvent(content string) {
 	s.Queue("<detached-process-result>\n" + content + "\n</detached-process-result>")
 }
 
+func (s *agent) React(emoji string) {
+	if s.session == nil {
+		return
+	}
+	msg := "[reacted " + emoji + " to your previous response]"
+	s.session.appendUserMessage(msg)
+	s.emit(Event{Role: "user", Content: msg})
+}
+
 func (s *agent) AgentName() string {
 	v := s.agentName
 	s.log.Debug("AgentName() = %q", v)

pkg/agent/types.go

@@ -167,6 +167,12 @@ type Core interface {
 	// context and emits it on the event bus. The agent sees it on its next
 	// turn; it appears in the chat log immediately.
 	InjectSystemEvent(content string)
+
+	// React records an emoji reaction to the most recent assistant response.
+	// The reaction is appended to session history as a lightweight user message
+	// and emitted to the chat bus, but does NOT trigger a new agent turn.
+	// The agent sees it as context on its next turn.
+	React(emoji string)
 }
 
 // DetachedInfo describes a detached process for external consumers.