docs: Add Common Pitfall #45 - 100% position sizing InsufficientCollateral

Documents the fix for InsufficientCollateral errors when using 100% position
sizing despite having correct account leverage.

Issue: Drift's margin calculation includes fees, slippage buffers, and rounding.
Using exact 100% of collateral leaves no room, causing $0.03-0.10 shortages.

Example: $85.55 collateral
- Bot tries: 100% = $85.55
- Margin needed: $85.58 (includes fees)
- Result: Rejected

Solution: Automatically apply 99% safety buffer when configured at 100%.
Result: $85.55 × 99% = $84.69 (leaves $0.86 safety margin)

Includes real incident details, code implementation, and math proof.
User's Drift UI screenshot proved account leverage WAS set correctly to 15x.

Related commit: 7129cbf

.github/copilot-instructions.md

@@ -1906,6 +1906,42 @@ trade.realizedPnL += actualRealizedPnL  // NOT: result.realizedPnL from SDK
     - **Git commit:** 528a0f4 "fix: Use Drift's actual entry price for breakeven SL"
     - **Lesson:** For critical price calculations (breakeven, TP, SL), always prefer on-chain data over cached database values
 
+45. **100% position sizing causes InsufficientCollateral (Fixed Nov 16, 2025):**
+    - **Symptom:** Bot configured for 100% position size gets InsufficientCollateral errors, but Drift UI can open same size position
+    - **Root Cause:** Drift's margin calculation includes fees, slippage buffers, and rounding - exact 100% leaves no room
+    - **Error details:**
+      ```
+      Program log: total_collateral=85547535 ($85.55)
+      Program log: margin_requirement=85583087 ($85.58)
+      Error: InsufficientCollateral (shortage: $0.03)
+      ```
+    - **Real incident (Nov 16, 01:50 CET):**
+      * Collateral: $85.55
+      * Bot tries: $1,283.21 notional (100% × 15x leverage)
+      * Drift UI works: $1,282.57 notional (has internal safety buffer)
+      * Difference: $0.64 causes rejection
+    - **Impact:** Bot cannot trade at full capacity despite account leverage correctly set to 15x
+    - **Fix:** Apply 99% safety buffer automatically when user configures 100% position size
+    ```typescript
+    // In config/trading.ts calculateActualPositionSize (line ~272):
+    let percentDecimal = configuredSize / 100
+    
+    // CRITICAL: Safety buffer for 100% positions
+    if (configuredSize >= 100) {
+      percentDecimal = 0.99
+      console.log(`⚠️ Applying 99% safety buffer for 100% position`)
+    }
+    
+    const calculatedSize = freeCollateral * percentDecimal
+    // $85.55 × 99% = $84.69 (leaves $0.86 for fees/slippage)
+    ```
+    - **Result:** $84.69 × 15x = $1,270.35 notional (well within margin requirements)
+    - **User experience:** Transparent - bot logs "Applying 99% safety buffer" when triggered
+    - **Why Drift UI works:** Has internal safety calculations that bot must replicate externally
+    - **Math proof:** 1% buffer on $85 = $0.85 safety margin (covers typical fees of $0.03-0.10)
+    - **Git commit:** 7129cbf "fix: Add 99% safety buffer for 100% position sizing"
+    - **Lesson:** When integrating with DEX protocols, never use 100% of resources - always leave safety margin for protocol-level calculations
+
 ## File Conventions
 
 - **API routes:** `app/api/[feature]/[action]/route.ts` (Next.js 15 App Router)