refactor: DiviWriteAbility base and shared DiviErrors (H4)

The five Divi write tools extend DiviWriteAbility (final permission and
execute, change() per tool, shared output schema). DiviErrors holds the
unknown-node, position and compose-error mapping; DiviContentComposer uses
the same mapping (ledger T9). ToolNamingTest skips abstract classes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

tests/Unit/Guides/ToolNamingTest.php

@@ -30,7 +30,7 @@ final class ToolNamingTest extends TestCase
 
             $class = 'LIT\\MCP\\' . str_replace(['/', '.php'], ['\\', ''], substr((string) $file, strlen($root . '/src/')));
             $reflection = new ReflectionClass($class);
-            if ($reflection->isInterface() || ! $reflection->implementsInterface(Ability::class)) {
+            if (! $reflection->isInstantiable() || ! $reflection->implementsInterface(Ability::class)) {
                 continue;
             }