docs: add regime-gate flip requirement to Tradier live runbook

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/TRADIER-LIVE-FLIP.md

@@ -29,6 +29,7 @@ Market data stays on Alpaca (unchanged). Rollback = set `TRADIER_ENV: sandbox`
 | `TRADING_EQUITY_FLOOR` | `270` | Kill switch at 0.90 x $300. |
 | `TRADING_MAX_RISK_DOLLARS` | `1000` (keep) | Inert at this equity (16% notional cap = ~$48 positions binds first); harmless. |
 | `TRADING_RISK_PER_TRADE_PCT` | `0.05` (keep) | $15 budget; the $48 position cap means actual risk/trade lands ~$2-8. |
+| `TRADING_REGIME_GATE_ENABLED` | `1` | The one lever that survived OOS validation (plan 011); flip-day requirement per LIVE-CUTOVER. |
 
 ## Before the flip — DB hygiene