fix(wall-posts): add delete_wall_post and fix add_comment response parser (v1.3.1)

- Fix add_comment response parser: corrected to a00.r.r.comment.commentId structure
- Fix add_comment error handling: return errors directly instead of warnings
- Add delete_wall_post: permanently delete wall posts via metadelete endpoint
- Add delete_wall_post to README.md Wall & Activities section
- Update SPEC.md with correct walladdComment response structure
- Update SPEC.md metadelete to include wall posts as supported type
- Update CHANGELOG.md with v1.3.1 bugfixes and additions
- Version bumped to 1.3.1 in pyproject.toml

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

SPEC.md

@@ -269,6 +269,7 @@ POST https://api.familywall.com/api/metadelete
 - Tasks: `task/<id>`
 - Rezepte: `recipe/<id>`
 - Essensplan-Einträge: `dish/<id>` und `meal/<id>`
+- Wall-Posts: `wall/<id>` (v1.3.1+)
 
 **Response:**
 ```
@@ -337,8 +338,8 @@ POST https://api.familywall.com/api/walladdComment
 
 **Response:**
 ```
-a00.r.r                     → Kommentar-Objekt
-  .metaId                   → neue Kommentar-ID
+a00.r.r.comment             → Kommentar-Objekt (nested)
+  .commentId                → neue Kommentar-ID
   .text                     → Kommentartext
   .creationDate             → Timestamp (ISO 8601)
 ```
@@ -346,7 +347,7 @@ a00.r.r                     → Kommentar-Objekt
 **Bekannte Einschränkungen:**
 - `mood` und `clientOpId` sind optional und werden ignoriert
 
-**Verifiziert am:** 2026-04-17 via Briefing und Integration
+**Verifiziert am:** 2026-04-17 via Briefing und Integration (Response-Struktur in v1.3.1 korrekt dokumentiert)
 
 ### `taskcategoryput` – Kategorie erstellen/aktualisieren
 POST https://api.familywall.com/api/taskcategoryput