p9: add workdir support to sessions

Sessions now accept workdir= at creation time:
  echo "new workdir=/home/lkn/src/myproject" > ctl

workdir is exposed as a r/w file in each session directory. Writing
to it updates the stored value (takes effect on next /agent switch,
since the execute server is constructed with the workdir at creation).

execute.Decl(workdir) and BuildAgentEnv(..., workdir) are wired
through so the sandbox and system prompt both use the correct path.

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

internal/p9/server.go

@@ -59,6 +59,7 @@ 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
@@ -209,7 +210,7 @@ func (s *Server) pathType(path string) string {
 			return ""
 		}
 		switch parts[2] {
-		case "ctl", "prompt", "chat", "reply", "backend", "agent", "model", "state":
+		case "ctl", "prompt", "chat", "reply", "backend", "agent", "model", "state", "workdir":
 			return "file"
 		}
 	}
@@ -439,6 +440,8 @@ func (s *Server) readFile(path string) string {
 		return sess.modelName + "\n"
 	case "state":
 		return sess.state + "\n"
+	case "workdir":
+		return sess.workdir + "\n"
 	}
 	return ""
 }
@@ -620,6 +623,11 @@ func (s *Server) handleWrite(path, input string) {
 			}
 			publish(ev)
 		})
+
+	case "workdir":
+		sess.mu.Lock()
+		sess.workdir = input
+		sess.mu.Unlock()
 	}
 }
 
@@ -645,12 +653,13 @@ func (s *Server) handleRootCtl(input string) {
 }
 
 // createSession parses key=value options and starts a new agent session.
-// Recognised keys: backend, model, agent. Unknown keys are rejected.
-// Example: new backend=ollama model=qwen3:8b agent=myagent
+// Recognised keys: backend, model, agent, workdir. Unknown keys are rejected.
+// Example: new backend=ollama model=qwen3:8b agent=myagent workdir=/home/lkn/src/myproject
 func (s *Server) createSession(args []string) error {
 	backendOverride := ""
 	modelOverride := ""
 	agentName := "default"
+	workdir := ""
 
 	for _, arg := range args {
 		k, v, ok := strings.Cut(arg, "=")
@@ -664,8 +673,10 @@ func (s *Server) createSession(args []string) error {
 			modelOverride = v
 		case "agent":
 			agentName = v
+		case "workdir":
+			workdir = v
 		default:
-			return fmt.Errorf("unknown option %q (valid: backend, model, agent)", k)
+			return fmt.Errorf("unknown option %q (valid: backend, model, agent, workdir)", k)
 		}
 	}
 
@@ -697,11 +708,11 @@ func (s *Server) createSession(args []string) error {
 	cfg, _ := config.Load(cfgPath) // nil cfg is handled by BuildAgentEnv
 
 	newDisp := tools.NewDispatcherFunc(map[string]func() tools.Server{
-		"execute": execute.Decl,
+		"execute": execute.Decl(workdir),
 		"file":    file.Decl,
 	})
 
-	env := agent.BuildAgentEnv(cfg, newDisp())
+	env := agent.BuildAgentEnv(cfg, newDisp(), workdir)
 	sessID := agent.NewSessionID()
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -711,6 +722,7 @@ func (s *Server) createSession(args []string) error {
 		AgentsDir:     s.agentsDir,
 		SessionsDir:   s.sessionsDir,
 		SessionID:     sessID,
+		WorkDir:       workdir,
 		Env:           env,
 		NewDispatcher: newDisp,
 	})
@@ -721,6 +733,7 @@ func (s *Server) createSession(args []string) error {
 		backendName: be.Name(),
 		agentName:   agentName,
 		modelName:   be.Model(),
+		workdir:     workdir,
 		ctx:         ctx,
 		cancel:      cancel,
 		state:       "idle",
@@ -823,6 +836,7 @@ func (s *Server) readDir(path string, offset uint64, count uint32) []byte {
 			{"backend", 0666},
 			{"agent", 0666},
 			{"model", 0666},
+			{"workdir", 0666},
 		}
 		for _, e := range files {
 			dirs = append(dirs, makeDir(e.name, sessPath+"/"+e.name, false, e.mode))
@@ -883,7 +897,7 @@ func (s *Server) makeStat(path string) plan9.Dir {
 			mode = 0200
 		case "chat", "reply", "state":
 			mode = 0444
-		case "backend", "agent", "model":
+		case "backend", "agent", "model", "workdir":
 			mode = 0666
 		default:
 			mode = 0444