makeStat: delegate file modes to backing tree

Remove hardcoded permission switch in makeStat and delegate to
the underlying FileTree.Stat() instead. This makes FileList in
sessionfile.go the single source of truth for session file modes.

server/server.go

@@ -1018,49 +1018,15 @@ func (s *Server) makeStat(path string) plan9.Dir {
 			mode = plan9.DMDIR | 0755
 		}
 	} else {
-		switch base {
-		case "ctl":
-			mode = 0222 // owner+group+other write
-		case "prompt", "fifo.in":
-			mode = 0022 // group+other write; owner cannot write (no self-prompt)
-		case "chat", "usage", "cost", "ctxsz", "models", "fifo.out", "offset", "statewait", "systemprompt":
-			mode = 0444
-		case "cfg":
-			mode = 0666
-		case "tail":
-			mode = 0555
-		default:
-			if path == "/backends" || path == "/help" {
-				mode = 0444
-			} else if isSessionFile(path) {
-				mode_, _ := session.FileMode(base)
-				mode = plan9.Perm(mode_)
-			} else if strings.HasPrefix(path, "/a/") {
-				mode = 0666
-			} else if strings.HasPrefix(path, "/p/") {
-				mode = 0664
-			} else if strings.HasPrefix(path, "/m/") {
-				mode = 0666
-			} else if path == "/sk/idx" {
-				mode = 0444
-			} else if strings.HasPrefix(path, "/sk/") {
-				mode = 0664
-			} else if strings.HasPrefix(path, "/s/") {
-				parts := strings.SplitN(strings.TrimPrefix(path, "/s/"), "/", 3)
-				if len(parts) == 3 && parts[1] == "t" && parts[2] == "idx" {
-					mode = 0400
-				} else if len(parts) == 3 && parts[1] == "t" {
-					mode = 0500
-				} else {
-					mode = 0444
-				}
-			} else if strings.HasPrefix(path, "/u/") {
-				mode = 0555
-			} else if strings.HasPrefix(path, "/x/") {
-				mode = 0555
+		// Delegate to the backing tree for file mode.
+		if tree, name := s.route(path); tree != nil {
+			if info, err := tree.Stat(name); err == nil {
+				mode = plan9.Perm(info.Mode())
 			} else {
 				mode = 0444
 			}
+		} else {
+			mode = 0444
 		}
 	}
 

session/sessionfile.go

@@ -23,10 +23,10 @@ var FileList = []struct {
 }{
 	{"plan", 0666, false, false},
 	{"ctl", 0200, false, true},
-	{"prompt", 0200, false, true},
-	{"fifo.in", 0200, false, true},
+	{"prompt", 0466, false, true},
+	{"fifo.in", 0222, false, true},
 	{"fifo.out", 0444, true, false},
-	{"chat", 0666, false, false},
+	{"chat", 0444, false, false},
 	{"offset", 0444, false, false},
 	{"cfg", 0666, false, false},
 	{"state", 0444, false, false},