refactor: remove all futures-related capabilities

Delete USDT-M futures support since the user's Binance API key does not
support futures trading. This simplifies the CLI to spot-only:

- Remove futures client wrapper (um_futures_client.py)
- Remove futures trade commands and close position logic
- Simplify account service to spot-only (no market_type field)
- Remove futures references from opportunity service
- Update README and tests to reflect spot-only architecture
- Bump version to 2.0.7

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -72,21 +72,16 @@ By default, CoinHunter prints human-friendly TUI tables. Add `--agent` to any co
 # Account
 coinhunter account overview
 coinhunter account overview --agent
-coinhunter account balances --spot --futures
-coinhunter account positions --spot
+coinhunter account balances
+coinhunter account positions
 
 # Market
 coinhunter market tickers BTCUSDT ETH/USDT sol-usdt
 coinhunter market klines BTCUSDT ETHUSDT --interval 1h --limit 50
 
-# Trade (Spot)
-coinhunter trade spot buy BTCUSDT --quote 100 --dry-run
-coinhunter trade spot sell BTCUSDT --qty 0.01 --type limit --price 90000
-
-# Trade (Futures)
-coinhunter trade futures buy BTCUSDT --qty 0.01 --dry-run
-coinhunter trade futures sell BTCUSDT --qty 0.01 --reduce-only
-coinhunter trade futures close BTCUSDT
+# Trade
+coinhunter trade buy BTCUSDT --quote 100 --dry-run
+coinhunter trade sell BTCUSDT --qty 0.01 --type limit --price 90000
 
 # Opportunities
 coinhunter opportunity portfolio
@@ -110,7 +105,7 @@ CoinHunter V2 uses a flat, direct architecture:
 | Layer | Responsibility | Key Files |
 |-------|----------------|-----------|
 | **CLI** | Single entrypoint, argument parsing | `cli.py` |
-| **Binance** | Thin API wrappers with unified error handling | `binance/spot_client.py`, `binance/um_futures_client.py` |
+| **Binance** | Thin API wrappers with unified error handling | `binance/spot_client.py` |
 | **Services** | Domain logic | `services/account_service.py`, `services/market_service.py`, `services/trade_service.py`, `services/opportunity_service.py` |
 | **Config** | TOML config, `.env` secrets, path resolution | `config.py` |
 | **Runtime** | Paths, TUI/JSON/compact output | `runtime.py` |