docs: add Squid Access Log Filtering Guide

docs/SQUID_LOG_FILTERING.md

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+# Squid Access Log Filtering Guide
+
+Quick reference for analyzing blocked domains in Squid access logs.
+
+## Log Location
+
+```bash
+# After awf execution, logs are preserved at:
+/tmp/squid-logs-<timestamp>/access.log
+
+# Find the latest log directory
+ls -lt /tmp/squid-logs-* | head -1
+```
+
+## Essential Filters
+
+### 1. Show Only Blocked Requests
+
+```bash
+sudo grep "TCP_DENIED" /tmp/squid-logs-*/access.log
+```
+
+**Output format:**
+```
+1761332530.123 172.30.0.20:35274 malicious.com:443 0.0.0.0:0 1.1 CONNECT 403 TCP_DENIED:HIER_NONE malicious.com:443 "curl/7.81.0"
+```
+
+### 2. Extract Blocked Domain Names Only
+
+```bash
+sudo grep "TCP_DENIED" /tmp/squid-logs-*/access.log | awk '{print $3}' | sort | uniq
+```
+
+**Output:**
+```
+malicious.com:443
+untrusted-api.example.org:443
+```
+
+### 3. Count Blocked Requests Per Domain
+
+```bash
+sudo grep "TCP_DENIED" /tmp/squid-logs-*/access.log | awk '{print $3}' | sort | uniq -c | sort -rn
+```
+
+**Output:**
+```
+25 malicious.com:443
+10 untrusted-api.example.org:443
+3 blocked-cdn.net:443
+```
+
+### 4. Show Blocked Requests with Timestamps
+
+```bash
+sudo grep "TCP_DENIED" /tmp/squid-logs-*/access.log | awk '{print strftime("%Y-%m-%d %H:%M:%S", $1), $3}'
+```
+
+**Output:**
+```
+2025-10-24 14:32:15 malicious.com:443
+2025-10-24 14:32:20 untrusted-api.example.org:443
+```
+
+### 5. Exclude Health Check Noise
+
+```bash
+sudo grep "TCP_DENIED" /tmp/squid-logs-*/access.log | grep -v "::1.*NONE_NONE"
+```
+
+## Common Patterns
+
+### Identify Source of Blocked Request
+
+```bash
+# Show client IP and domain for blocked requests
+sudo grep "TCP_DENIED" /tmp/squid-logs-*/access.log | awk '{print "Client:", $2, "→ Blocked:", $3}'
+```
+
+**Output:**
+```
+Client: 172.30.0.20:35274 → Blocked: malicious.com:443
+Client: 172.30.0.2:54610 → Blocked: untrusted-api.example.org:443
+```
+
+### Filter by Time Range
+
+```bash
+# Show blocked requests in last 5 minutes of log
+sudo awk -v cutoff=$(date -d '5 minutes ago' +%s) '$1 > cutoff && /TCP_DENIED/' /tmp/squid-logs-*/access.log
+```
+
+### Show User-Agent for Blocked Requests
+
+```bash
+# Extract User-Agent (last field in quotes)
+sudo grep "TCP_DENIED" /tmp/squid-logs-*/access.log | sed 's/.*"\(.*\)"/\1/'
+```
+
+## Decision Codes Reference
+
+| Code | Meaning | Action |
+|------|---------|--------|
+| `TCP_DENIED:HIER_NONE` | Domain not in allowlist | **BLOCKED** ❌ |
+| `TCP_TUNNEL:HIER_DIRECT` | Domain allowed, tunnel established | **ALLOWED** ✅ |
+| `NONE_NONE:HIER_NONE` | Connection error (no HTTP headers) | **N/A** (health checks) |
+
+## Quick Diagnostics
+
+### 1. "Why was my command blocked?"
+
+```bash
+# Find all denied domains from last run
+sudo grep "TCP_DENIED" $(ls -t /tmp/squid-logs-*/access.log | head -1) | awk '{print $3}' | sort -u
+```
+
+### 2. "Which domains should I add to allowlist?"
+
+```bash
+# Show most frequently blocked domains
+sudo grep "TCP_DENIED" /tmp/squid-logs-*/access.log | awk '{print $3}' | sed 's/:443$//' | sort | uniq -c | sort -rn | head -10
+```
+
+### 3. "Is my allowlist working?"
+
+```bash
+# Compare allowed vs blocked counts
+echo "Allowed: $(sudo grep -c "TCP_TUNNEL" /tmp/squid-logs-*/access.log)"
+echo "Blocked: $(sudo grep -c "TCP_DENIED" /tmp/squid-logs-*/access.log)"
+```
+
+## Notes
+
+- All commands require `sudo` because log files are owned by the `proxy` user from the container
+- Use `$(ls -t /tmp/squid-logs-*/access.log | head -1)` to automatically target the latest log
+- Timestamps are Unix epoch seconds (use `date -d @<timestamp>` to convert)
+- Port `:443` indicates HTTPS traffic (most common)
+- Client IPs: `172.30.0.20` = copilot container, `172.30.0.2` = spawned containers