feat: add ai agent http agent

texas_holdem/prompts/observation.md

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+# OBSERVATION (your turn to act)
+
+It is your turn. Read the state below and respond with a single JSON
+object: `{{"action": "<name>", "amount": <int>}}`. Pick only from the
+listed legal actions.
+
+## Hand state
+
+- Hand number: {hand_number}
+- Street: {street}
+- You are: player_id={player_id}, name={player_name}, seat={seat}
+- Button seat: {button_seat}
+- Pot size: {pot}
+- To call: {to_call}
+- Min raise to: {min_raise_to}
+- Amount semantics for bet/raise: {amount_mode} (the integer is the
+  target total street bet, NOT the delta on top of your current bet)
+
+## Cards
+
+- Your hole cards: {hole_cards}
+- Community board: {board}
+
+## Players at the table
+
+{players_block}
+
+## Action history (this hand)
+
+{action_history_block}
+
+## Legal actions
+
+{legal_actions_block}
+
+Respond NOW with one JSON line and nothing else.