p9: remove dead sess.workdir field

Now that reads go through sess.core.WorkDir(), the cached workdir field
on the session struct serves no purpose. Remove it and the write that
kept it in sync.

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

internal/p9/server.go

@@ -63,7 +63,6 @@ type session struct {
 	// state is the current agent state: "idle", "thinking", "calling: <tool>"
 	state     string
 	modelName string
-	workdir   string
 	// reply holds only the assistant text from the most recently completed
 	// turn. Cleared when a new prompt is submitted. Read-only; 0444.
 	reply     []byte
@@ -769,11 +768,7 @@ 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()
 	}
 }
 
@@ -880,7 +875,6 @@ func (s *Server) createSession(args []string) error {
 		backendName: be.Name(),
 		agentName:   agentName,
 		modelName:   be.Model(),
-		workdir:     workdir,
 		ctx:         ctx,
 		cancel:      cancel,
 		state:       "idle",