docs: add "static" whitelist example to shell

Fixed regex matching security issue where dots in domain names were treated
as regex wildcards. Now uses grep -Fxq for literal string matching:
- F: Fixed strings (no regex)
- x: Match whole lines exactly
- q: Quiet mode

This prevents domains like "apiXgithubYcom" from matching "api.github.com".

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: ammario

docs/guide/rule-engines/shell.md

@@ -88,8 +88,8 @@ if [ ! -f "$WHITELIST_FILE" ]; then
     exit 1
 fi
 
-# Check if current host is in the whitelist
-if grep -q "^${HTTPJAIL_HOST}$" "$WHITELIST_FILE"; then
+# Check if current host is in the whitelist (exact match)
+if grep -Fxq "$HTTPJAIL_HOST" "$WHITELIST_FILE"; then
     exit 0  # Allow
 else
     echo "Host $HTTPJAIL_HOST not in whitelist"