botfather: /start <bot> opens that bot's menu

The "Manage Bot" button on a bot's profile deep-links to @BotFather with
start=<bot username>. parseBotCommand dropped the argument, so /start <bot>
just replied with the generic greeting instead of the per-bot menu.

Route "/start <arg>" to the bot's "What do you want to do?" screen (same as
/mybots then tapping the bot) when <arg> names one of the sender's own bots by
username or id; empty or unknown args keep the greeting.

internal/app/bots/botfather.go

@@ -278,6 +278,13 @@ func (s *Service) handleBotFather(ctx context.Context, userID int64, msg domain.
 	if cmd, ok := parseBotCommand(text); ok {
 		inValueStep := found && state.Step == botFatherStepValue
 		if !inValueStep || botFatherGlobalCommands[cmd] {
+			// "/start <bot>" (the "Manage Bot" deep link) jumps straight to that
+			// bot's menu, like /mybots then tapping the bot.
+			if cmd == "start" {
+				if arg := botCommandArg(text); arg != "" {
+					return s.handleBotFatherStart(ctx, userID, arg)
+				}
+			}
 			return s.handleBotFatherCommand(ctx, userID, cmd)
 		}
 	}
@@ -372,6 +379,41 @@ func (s *Service) stepPrompt(state domain.BotChatState) botReply {
 	}
 }
 
+// handleBotFatherStart answers "/start <arg>". When <arg> names one of the
+// user's own bots (by username or numeric id) it opens that bot's menu - the
+// same "What do you want to do?" screen as /mybots then tapping the bot, which
+// is what the "Manage Bot" button on a bot's profile links to. An empty or
+// unknown arg falls back to the plain greeting.
+func (s *Service) handleBotFatherStart(ctx context.Context, userID int64, arg string) botReply {
+	_ = s.bots.DeleteBotChatState(ctx, domain.BotFatherUserID, userID)
+	want := strings.ToLower(strings.TrimPrefix(strings.TrimSpace(arg), "@"))
+	if want == "" {
+		return botReply{Text: botFatherHelpText}
+	}
+	owned, err := s.ownedBots(ctx, userID)
+	if err != nil {
+		s.log.Error("botfather: list bots for start payload", zap.Int64("user_id", userID), zap.Error(err))
+		return internalReply()
+	}
+	for _, b := range owned {
+		if strings.EqualFold(b.user.Username, want) || strconv.FormatInt(b.user.ID, 10) == want {
+			state := domain.BotChatState{
+				BotUserID: domain.BotFatherUserID,
+				UserID:    userID,
+				Command:   mybotsCommand,
+				Step:      mybotsStepMenu,
+				Draft:     map[string]string{},
+			}
+			reply := s.myBotsBotMenu(&state, b)
+			if !s.saveMyBotsState(ctx, state) {
+				return internalReply()
+			}
+			return reply
+		}
+	}
+	return botReply{Text: botFatherHelpText}
+}
+
 func (s *Service) handleBotFatherCommand(ctx context.Context, userID int64, cmd string) botReply {
 	switch cmd {
 	case "start", "help":
@@ -1161,3 +1203,16 @@ func parseBotCommand(text string) (string, bool) {
 	}
 	return strings.ToLower(cmd), true
 }
+
+// botCommandArg returns the trimmed argument after a leading "/cmd", e.g.
+// "/start my_bot" -> "my_bot". Empty when there is no argument.
+func botCommandArg(text string) string {
+	text = strings.TrimSpace(text)
+	if !strings.HasPrefix(text, "/") {
+		return ""
+	}
+	if i := strings.IndexAny(text, " \t\n"); i >= 0 {
+		return strings.TrimSpace(text[i+1:])
+	}
+	return ""
+}

internal/app/bots/mybots_test.go

@@ -202,6 +202,40 @@ func TestMyBotsBotMenuAndBack(t *testing.T) {
 	}
 }
 
+func TestBotFatherStartWithBotOpensItsMenu(t *testing.T) {
+	svc, users, _, messages := newTestService(t)
+	owner := newOwner(t, users, "+2011")
+	makeBots(t, svc, owner.ID, 2)
+
+	// "/start <bot>" is the "Manage Bot" deep link: it lands on the per-bot menu.
+	body := sendToBotFather(t, svc, messages, owner, "/start mb0_bot")
+	if !strings.Contains(body, "@mb0_bot") || !strings.Contains(body, "What do you want to do?") {
+		t.Fatalf("/start mb0_bot reply = %q", body)
+	}
+	menu := botFatherUserReply(t, messages, owner.ID)
+	for _, want := range []string{"API Token", "Edit Bot", "Bot Settings", "Delete Bot", "Back to bots"} {
+		if !mybotsHasButton(menu, want) {
+			t.Fatalf("start menu missing %q: %+v", want, menu.ReplyMarkup)
+		}
+	}
+	// The buttons are live (state was saved), so Edit Bot works from here.
+	_, edit := pressBotFather(t, svc, messages, owner.ID, "Edit Bot")
+	if !strings.Contains(edit.Body, "@mb0_bot") {
+		t.Fatalf("edit menu after /start = %q", edit.Body)
+	}
+
+	// A leading @ and an unknown/foreign bot fall back to the greeting.
+	if body := sendToBotFather(t, svc, messages, owner, "/start @mb1_bot"); !strings.Contains(body, "@mb1_bot") {
+		t.Fatalf("/start @mb1_bot reply = %q", body)
+	}
+	if body := sendToBotFather(t, svc, messages, owner, "/start not_a_real_bot"); !strings.Contains(body, "create a new bot") {
+		t.Fatalf("/start unknown reply = %q, want greeting", body)
+	}
+	if body := sendToBotFather(t, svc, messages, owner, "/start"); !strings.Contains(body, "create a new bot") {
+		t.Fatalf("bare /start reply = %q, want greeting", body)
+	}
+}
+
 func TestMyBotsTokenAndRevoke(t *testing.T) {
 	svc, users, bots, messages := newTestService(t)
 	owner := newOwner(t, users, "+2003")