feat(ECR-021): AICoreConfig SystemPrompt 只读并 Closed

新增 system_prompts 目录、admin.ai_config.read 与 admin-h5「AI」页；本切片不改运行时 Prompt、禁写发布/UGC/真支付。

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/CONTRACT_DIFF/ECR-021.yaml

@@ -0,0 +1,23 @@
+ecr: ECR-021
+capability: AICoreConfig
+bounded_context: Ask_Ops
+parent: WAVE0-FROZEN
+predecessor: ECR-020
+change:
+  type: additive
+breaking_change: false
+migration_required: true
+compatibility_notes: >
+  Adds system_prompts catalog with ask_default seed and read-only admin APIs.
+  Does not change Ask runtime prompt assembly in this slice.
+
+apis:
+  - method: GET
+    path: /api/v1/admin/ai/system-prompts
+    change: added
+  - method: GET
+    path: /api/v1/admin/ai/system-prompts/{id}
+    change: added
+perms:
+  - code: admin.ai_config.read
+    change: added