fs: move cwd to ctl command, remove cwd file

/cwd returns current directory, /cwd X changes it (with stat check).
One fewer file in the agent namespace.

data/scripts/o

@@ -33,7 +33,7 @@ ROOT_ENTRIES="session bypass agents backends models help ctl eventwait complete
 SESSION_FILES="env paused connected ctl name id bypass"
 
 # Agent-level files (exist under session/{name}/agent/{name}/)
-AGENT_FILES="prompt fifo chat chat.raw statewait log plan cfg ctl cwd stats systemprompt tools name id proc"
+AGENT_FILES="prompt fifo chat chat.raw statewait log plan cfg ctl stats systemprompt tools name id proc"
 
 is_root_path() {
     local path="$1"

fs/handlers.go

@@ -731,30 +731,17 @@ func requestAgentCtl(ctx HandlerCtx, data []byte) ([]byte, error) {
 		if len(args) == 0 {
 			return []byte(ctx.Agent.Cwd() + "\n"), nil
 		}
-		ctx.Agent.SetCWD(paths.ExpandHome(strings.Join(args, " ")))
+		dir := paths.ExpandHome(strings.Join(args, " "))
-		return []byte(strings.Join(args, " ") + "\n"), nil
+		if _, err := os.Stat(dir); err != nil {
+			return nil, fmt.Errorf("cwd: %w", err)
+		}
+		ctx.Agent.SetCWD(dir)
+		return []byte(dir + "\n"), nil
 	default:
 		return nil, fmt.Errorf("unknown ctl command: %s", cmd)
 	}
 }
 
-func readAgentCwd(ctx HandlerCtx) ([]byte, error) {
-	return []byte(ctx.Agent.Cwd() + "\n"), nil
-}
-
-func writeAgentCwd(ctx HandlerCtx, data []byte) error {
-	input := strings.TrimSpace(string(data))
-	if input == "" {
-		return nil
-	}
-	input = paths.ExpandHome(input)
-	if _, err := os.Stat(input); err != nil {
-		return fmt.Errorf("cwd: %w", err)
-	}
-	ctx.Agent.SetCWD(input)
-	return nil
-}
-
 func readAgentStats(ctx HandlerCtx) ([]byte, error) {
 	s := "usage=" + ctx.Agent.UsageStr() + "\n" +
 		"cost=" + ctx.Agent.CostStr() + "\n" +

fs/spec.go

@@ -269,12 +269,6 @@ FileNode("generate", 0666,
 						Request(requestAgentCtl),
 					),
 
-					FileNode("cwd", 0666,
-						Doc("Agent working directory. Read: current path. Write: change directory."),
-						Read(readAgentCwd),
-						Write(writeAgentCwd),
-					),
-
 					FileNode("stats", 0444,
 						Doc("Agent statistics. Key=value lines: usage=, cost=, ctxsz=."),
 						Read(readAgentStats),