agent: inject detached process output on exit

When a detached process exits, its output (last 2KB of ring buffer)
is queued as a prompt wrapped in <detached-process-result> tags.
The agent sees it on its next turn (or immediately if idle via Queue).

- InjectSystemEvent(content) added to Core interface
- SetOnExit(fn) added to execute.Server for external hook wiring

pkg/agent/core.go

@@ -540,6 +540,10 @@ func (s *agent) DismissDetached(pid int) bool {
 	return false
 }
 
+func (s *agent) InjectSystemEvent(content string) {
+	s.Queue("<detached-process-result>\n" + content + "\n</detached-process-result>")
+}
+
 func (s *agent) AgentName() string {
 	v := s.agentName
 	s.log.Debug("AgentName() = %q", v)

pkg/agent/types.go

@@ -162,6 +162,11 @@ type Core interface {
 
 	// DismissDetached removes an exited process from the list.
 	DismissDetached(pid int) bool
+
+	// InjectSystemEvent appends a system-originated message to the session
+	// 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)
 }
 
 // DetachedInfo describes a detached process for external consumers.

pkg/tools/execute/server.go

@@ -89,6 +89,9 @@ func WithOnPreDispatch(fn func()) Option { return func(s *Server) { s.OnPreDispa
 // WithOnClose registers a hook called during Close.
 func WithOnClose(fn func()) Option { return func(s *Server) { s.OnClose = fn } }
 
+// SetOnExit sets the hook called when a detached process exits.
+func (e *Server) SetOnExit(fn func(pid, exitCode int)) { e.OnExit = fn }
+
 // WithOnEnvSet registers a hook called each time SetEnv is called.
 func WithOnEnvSet(fn func(key, value string)) Option { return func(s *Server) { s.OnEnvSet = fn } }