feat: init nofxi - AI Trading Agent module

NOFXi — Your AI Trading Agent, built on NOFX.

Architecture:
- Agent Core: intent routing, conversation memory, trade confirmation
- Memory: SQLite (trades, conversations, preferences, strategies)
- Thinking: LLM client (OpenAI/claw402/DeepSeek compatible)
- Execution: Bridge to NOFX trader engine (9 exchanges)
- Perception: Market monitor, price alerts, anomaly detection
- Interaction: Telegram bot + REST API + OpenAI-compatible endpoint

Features:
- Natural language trading (中英文)
- /buy /sell /analyze /watch /alert /positions /balance
- Daily P/L reports, portfolio risk checks
- Trade confirmation flow (safety first)
- Price polling and alert notifications

nofxi/internal/execution/execution.go

@@ -0,0 +1,47 @@
+// Package execution implements the Execution Layer.
+//
+// Phase 2 — stubs for now.
+//
+// Planned capabilities:
+// - Multi-exchange order execution (via NOFX engine)
+// - Position management (open/close/adjust)
+// - Stop-loss and take-profit management
+// - x402 payment integration via Claw402
+// - Safe mode and risk controls
+package execution
+
+// Executor is the trade execution interface.
+type Executor interface {
+	// PlaceOrder places a trade order on the exchange.
+	PlaceOrder(exchange, symbol, side string, price, quantity float64) (string, error)
+
+	// ClosePosition closes an existing position.
+	ClosePosition(exchange, symbol string) error
+
+	// GetPositions returns current open positions.
+	GetPositions(exchange string) ([]Position, error)
+
+	// GetBalance returns account balance.
+	GetBalance(exchange string) (*Balance, error)
+}
+
+// Position represents an open position.
+type Position struct {
+	Exchange   string  `json:"exchange"`
+	Symbol     string  `json:"symbol"`
+	Side       string  `json:"side"`
+	Size       float64 `json:"size"`
+	EntryPrice float64 `json:"entry_price"`
+	MarkPrice  float64 `json:"mark_price"`
+	PnL        float64 `json:"pnl"`
+	Leverage   float64 `json:"leverage"`
+}
+
+// Balance represents account balance.
+type Balance struct {
+	Exchange   string  `json:"exchange"`
+	Total      float64 `json:"total"`
+	Available  float64 `json:"available"`
+	InPosition float64 `json:"in_position"`
+	Currency   string  `json:"currency"`
+}