p9: wire SetWorkDir into workdir write handler

Validate the path and propagate the change to the agent core (which
updates the execute server and rebuilds the system prompt) before
updating the session's cached workdir field. Return early on error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/p9/server.go

@@ -767,6 +767,10 @@ func (s *Server) handleWrite(path, input string) {
 		})
 
 	case "workdir":
+		if err := sess.core.SetWorkDir(input); err != nil {
+			fmt.Fprintf(os.Stderr, "olliesrv: workdir: %v\n", err)
+			return
+		}
 		sess.mu.Lock()
 		sess.workdir = input
 		sess.mu.Unlock()