docs: Add Common Pitfall #35 - phantom trades need exitReason

- Documented bug where phantom auto-closure sets status='phantom' but left exitReason=NULL
- Startup validator only checks exitReason, not status field
- Ghost positions created false runner stop loss alerts (232% size mismatch)
- Fix: MUST set exitReason when closing phantom trades
- Manual cleanup: UPDATE Trade SET exitReason='manual' WHERE status='phantom' AND exitReason IS NULL
- Verified: System now shows 'Found 0 open trades' after cleanup

.github/copilot-instructions.md

@@ -1426,6 +1426,28 @@ trade.realizedPnL += actualRealizedPnL  // NOT: result.realizedPnL from SDK
     - **Lesson:** Always validate input data for financial calculations, especially when data might not exist yet
     - **Monitoring:** Watch logs for "🔍 Flip-flop price check: $X → $Y = Z%" to verify correct calculations
 
+35. **Phantom trades need exitReason for cleanup (CRITICAL - Fixed Nov 15, 2025):**
+    - **Symptom:** Position Manager keeps restoring phantom trade on every restart, triggers false runner stop loss alerts
+    - **Root Cause:** Phantom auto-closure sets `status='phantom'` but leaves `exitReason=NULL`
+    - **Bug:** Startup validator checks `exitReason !== null` (line 122 of init-position-manager.ts), ignores status field
+    - **Consequence:** Phantom trade with exitReason=NULL treated as "open" and restored to Position Manager
+    - **Real incident:** Nov 14 phantom trade (cmhy6xul20067nx077agh260n) caused 232% size mismatch, hundreds of false "🔴 RUNNER STOP LOSS" alerts
+    - **Fix:** When auto-closing phantom trades, MUST set exitReason:
+    ```typescript
+    // In app/api/trading/execute/route.ts (phantom detection):
+    await updateTradeExit({
+      tradeId: trade.id,
+      exitPrice: currentPrice,
+      exitReason: 'manual', // CRITICAL: Must set exitReason for cleanup
+      realizedPnL: actualPnL,
+      status: 'phantom'
+    })
+    ```
+    - **Manual cleanup:** If phantom already exists: `UPDATE "Trade" SET "exitReason" = 'manual' WHERE status = 'phantom' AND "exitReason" IS NULL`
+    - **Impact:** Without exitReason, phantom trades create ghost positions that trigger false alerts and pollute monitoring
+    - **Verification:** After restart, check logs for "Found 0 open trades" (not "Found 1 open trades to restore")
+    - **Lesson:** status field is for classification, exitReason is for lifecycle management - both must be set on closure
+
 ## File Conventions
 
 - **API routes:** `app/api/[feature]/[action]/route.ts` (Next.js 15 App Router)