refactor: WriteFailed named constructors, Rollback runner, Errors read-back helpers (H6)

- WriteFailed::rolledBack()/partial() replace the 11-parameter constructor (now
  private); the hint/state of either error are plain $hint/$state.
- Rollback::attempt(...steps) runs every undo step and collects failure messages;
  used by ContentCreate::discard, ContentUpdate::rollBack, MediaUpdate::rollBack and
  DiviSaver::restore.
- Errors::clientMessage/codeOf/fieldsOf replace DiviReplaceTextAbility::reason,
  the JSON digging in MediaUpdateAbility::failure and ContentCreateAbility::fields.
- Errors docblock documents the exception/WP_Error split.
- ContentCreateAbility::discard takes the client reason as a parameter (both callers
  still pass REASON_EXTRAS; changing the Divi text needs a ruling).

Error payloads unchanged.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/Abilities/Rollback.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Abilities;
+
+use Throwable;
+
+/**
+ * Runs the undo steps of a failed write. Every step runs, even after an earlier one failed;
+ * a step fails by throwing. The caller turns the collected messages into the log detail and
+ * picks WriteFailed::rolledBack() (no failures) or WriteFailed::partial().
+ */
+final class Rollback
+{
+    /**
+     * @param callable(): mixed ...$steps In the order they must run.
+     * @return list<string> The messages of the steps that threw, in step order. Never throws.
+     */
+    public static function attempt(callable ...$steps): array
+    {
+        $failures = [];
+        foreach ($steps as $step) {
+            try {
+                $step();
+            } catch (Throwable $e) {
+                $failures[] = $e->getMessage();
+            }
+        }
+
+        return $failures;
+    }
+}