Fix shop action costs and pass semantics.

Only buying costs gold; selling and trading (Triple) are free. Pass is
now a final action, legal only at or under the pet limit: it forfeits
remaining gold and ends that player's shopping for the round, with the
shop closing once everyone has passed. That makes the separate cleanup
phase unreachable (you sell down in-shop before passing), so it is
removed. The client asks for confirmation before passing, and the bot
knows buys are the only coin sink, when passing is legal, and that it
must sell down before it can pass.

README.md

@@ -48,18 +48,20 @@ Set via environment or a `.env` file (see `.env.example`):
 
 Six rounds, each with its own shop tier deck. Per round:
 
-1. **Shop** — each player has 3 gold and players alternate actions, 1 gold
-   each: **buy** one of 4 face-up cards; **sell** any number of hand cards
-   (each becomes an 🍎 apple, +1 power food, and Sell effects fire); or
-   **trade in** 3 same-suit pets (the Triple action) to pick 1 of the top 2
-   cards of the next tier's deck — Triple effects fire on the traded cards
-   and the received pet's Buy effect fires. Passing forfeits remaining gold.
-2. **Cleanup** — anyone holding more than 5 pets must sell down to 5.
-3. **Arrange** — Battle Prep effects fire first (e.g. Giraffe hands out
+1. **Shop** — each player has 3 gold and players alternate actions: **buy**
+   one of 4 face-up cards (1 gold — the only action that costs anything);
+   **sell** any number of hand cards for free (each becomes an 🍎 apple, +1
+   power food, and Sell effects fire); or **trade in** 3 same-suit pets (the
+   Triple action, also free) to pick 1 of the top 2 cards of the next tier's
+   deck — Triple effects fire on the traded cards and the received pet's Buy
+   effect fires. **Passing** is a final action, only legal at 5 pets or
+   fewer: it forfeits remaining gold and ends that player's shopping for the
+   round. The shop closes once everyone has passed.
+2. **Arrange** — Battle Prep effects fire first (e.g. Giraffe hands out
    apples), then players secretly order their decks. Food cards apply to the
    next pet after them; trailing foods are wasted. A pet only benefits from
    its last-applied **perk** (e.g. Honey, Garlic).
-4. **Battle** — automatic stack machine. Cards reveal off the top of each
+3. **Battle** — automatic stack machine. Cards reveal off the top of each
    deck until a pet is in play. Play effects fire on reveal (rocks roll a d6
    with faces 0/0/1/1/2/2 and hit the opposing pet before the clash; Skunk
    strips foods; Wolverine steals apples; Chili mills the enemy deck). The