docs: Document BlockedSignal data contamination from old v9 alerts

- Discovery: All TradingView alerts (5min/15min/1H/4H/Daily) attached to OLD v9 version
- Impact: 11,429 records from wrong indicator settings (confirmBars=0 vs current)
- Solution: Marked as DATA_COLLECTION_OLD_V9_VERSION to prevent analysis contamination
- Exception: 1-minute data (11,398) kept as DATA_COLLECTION_ONLY (unaffected)
- Fresh data from corrected alerts will use DATA_COLLECTION_ONLY going forward
- Old data preserved for historical reference, clearly marked

.github/copilot-instructions.md

@@ -635,6 +635,15 @@ docs/COMMON_PITFALLS.md
 
 **Purpose:** Track exact minute-by-minute price movements for blocked signals to determine EXACTLY when TP1/TP2 would have been hit
 
+**CRITICAL: Data Contamination Discovery (Dec 5, 2025):**
+- **Problem:** All TradingView alerts (5min, 15min, 1H, 4H, Daily) were attached to OLD v9 version with different settings
+- **Impact:** 11,429 BlockedSignal records collected from wrong indicator parameters (confirmBars=0 instead of current v9)
+- **Solution:** Marked contaminated data with `blockReason='DATA_COLLECTION_OLD_V9_VERSION'`
+- **Exception:** 1-minute data (11,398 records) kept as `DATA_COLLECTION_ONLY` - not affected by alert version issue (pure market data sampling)
+- **SQL Filter:** Exclude old data: `WHERE blockReason != 'DATA_COLLECTION_OLD_V9_VERSION'`
+- **Fresh Start:** New signals from corrected alerts will use `blockReason='DATA_COLLECTION_ONLY'`
+- **Database State:** Old data preserved for historical reference, clearly marked to prevent analysis contamination
+
 **Critical Optimization (Dec 2, 2025):**
 - **Original Threshold:** 30 minutes (arbitrary, inefficient)
 - **User Insight:** "we have 1 minute data, so use it"