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/Integrations/Divi/Pages/DiviSaver.php

@@ -6,6 +6,7 @@ namespace LIT\MCP\Integrations\Divi\Pages;
 
 use LIT\MCP\Abilities\Errors;
 use LIT\MCP\Abilities\FollowUp;
+use LIT\MCP\Abilities\Rollback;
 use LIT\MCP\Abilities\WriteFailed;
 use LIT\MCP\Integrations\WordPress\PostGuard;
 use LIT\MCP\Integrations\WordPress\RevisionPoint;
@@ -78,14 +79,12 @@ class DiviSaver
 
         if ($result instanceof WP_Error || $result === 0) {
             $message = $result instanceof WP_Error ? $result->get_error_message() : 'returned 0';
-            throw new WriteFailed(
+            throw WriteFailed::rolledBack(
                 self::REASON,
-                true,
-                $postId,
-                $revisionId,
-                null,
-                sprintf('wp_update_post(%d) refused: %s', $postId, $message),
-                rolledBackState: self::keeping($revisionId)
+                detail: sprintf('wp_update_post(%d) refused: %s', $postId, $message),
+                state: self::keeping($revisionId),
+                postId: $postId,
+                revisionId: $revisionId
             );
         }
 
@@ -172,7 +171,7 @@ class DiviSaver
     private function restore(int $postId, string $before, ?int $revisionId, Throwable $cause): never
     {
         $detail = sprintf('Saving Divi content of post %d failed: %s', $postId, $cause->getMessage());
-        try {
+        $failures = Rollback::attempt(function () use ($postId, $before): void {
             $current = get_post($postId);
             if ($current instanceof WP_Post && PostGuard::missedSchedule($current)) {
                 throw new RuntimeException('restoring would publish a missed schedule');
@@ -185,19 +184,19 @@ class DiviSaver
             if ($restored instanceof WP_Error || $restored === 0) {
                 throw new RuntimeException('previous content could not be written back');
             }
-        } catch (Throwable $e) {
-            $detail .= '; rollback: ' . $e->getMessage();
-            throw new WriteFailed(self::REASON, false, $postId, $revisionId, $cause, $detail);
+        });
+        if ($failures !== []) {
+            $detail .= '; rollback: ' . $failures[0];
+            throw WriteFailed::partial(self::REASON, $cause, $detail, $postId, $revisionId);
         }
 
-        throw new WriteFailed(
+        throw WriteFailed::rolledBack(
             self::REASON,
-            true,
-            $postId,
-            $revisionId,
             $cause,
             $detail,
-            rolledBackState: self::keeping($revisionId)
+            state: self::keeping($revisionId),
+            postId: $postId,
+            revisionId: $revisionId
         );
     }