docs: consolidate documentation into organized structure

- Move 12 markdown files from root to docs/ subdirectories
- Organize documentation by category:
  • docs/troubleshooting/ (1 file)  - Technical troubleshooting guides
  • docs/deployment/      (4 files) - Deployment and security documentation
  • docs/guides/          (3 files) - Feature-specific guides
  • docs/planning/        (4 files) - Planning and improvement proposals

Root directory cleanup:
- Reduced from 16 to 4 markdown files in root
- Only essential project files remain:
  • CLAUDE.md (AI instructions)
  • README.md (Main project readme)
  • CLEANUP_PLAN.md (Current cleanup plan)
  • SRC_STRUCTURE_IMPROVEMENTS.md (Structure improvements)

This improves:
✅ Documentation discoverability
✅ Logical organization by purpose
✅ Clean root directory
✅ Better maintainability

src/Framework/Queue/Exceptions/InvalidDeadLetterQueueNameException.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Framework\Queue\Exceptions;
+
+use App\Framework\Exception\FrameworkException;
+use App\Framework\Exception\ErrorCode;
+
+final class InvalidDeadLetterQueueNameException extends FrameworkException
+{
+    public static function tooShort(string $name, int $minLength): self
+    {
+        return self::create(
+            ErrorCode::VAL_BUSINESS_RULE_VIOLATION,
+            "Dead letter queue name '{$name}' is too short. Minimum length is {$minLength} characters."
+        )->withData([
+            'name' => $name,
+            'actual_length' => strlen($name),
+            'min_length' => $minLength
+        ]);
+    }
+
+    public static function tooLong(string $name, int $maxLength): self
+    {
+        return self::create(
+            ErrorCode::VAL_BUSINESS_RULE_VIOLATION,
+            "Dead letter queue name '{$name}' is too long. Maximum length is {$maxLength} characters."
+        )->withData([
+            'name' => $name,
+            'actual_length' => strlen($name),
+            'max_length' => $maxLength
+        ]);
+    }
+
+    public static function invalidFormat(string $name, string $pattern): self
+    {
+        return self::create(
+            ErrorCode::VAL_BUSINESS_RULE_VIOLATION,
+            "Dead letter queue name '{$name}' contains invalid characters. Only alphanumeric characters, underscores, hyphens, and dots are allowed."
+        )->withData([
+            'name' => $name,
+            'valid_pattern' => $pattern
+        ]);
+    }
+}