feat(domains): identity, billing, task services and domain placeholders

Billing implements reserve -> commit -> refund against bl_quota and bl_ledger
with row-level locking and idempotency keys; the refund commits on its own
transaction so a failed request cannot silently keep a user's quota. Task
implements the unified task centre over the state machine. Identity issues and
resolves users. Remaining domains are placeholders whose READMEs state what
belongs there and what must not.

packages/memory/README.md

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+# `packages/memory/` — 记忆系统（占位）
+
+架构 §8.4。与 `packages/profile` 配合：`profile` 管表，这里管策略。
+
+## 读（注入）
+
+```
+get_memory_snapshot(user_id, budget_tokens)
+  → 优先级：L0 身份 > L1 画像 > L2 显式偏好 > L5 衣橱 > L3 事件
+  → 过滤 status=active 且 confidence >= 阈值
+  → 裁剪到预算内（≤ 500 token，FR-MEM-04）→ 缓存（hash 变更才重建）
+```
+
+## 写
+
+| 来源 | 置信度 | 行为 |
+| --- | --- | --- |
+| 显式（用户明确说的） | 0.95 | 立即生效，回执告知「已记住」 |
+| 报告产出 | 诊断置信度 | 写 L1 |
+| 行为推断（隐式） | 累积证据 | 达阈值生成 `pending_confirm`，确认后才 active |
+
+## 铁律
+
+**用户在「我的记忆」页面的编辑拥有最高优先级，且不可被自动覆盖**
+（FR-MEM-05/07）。所有变更写 `pf_memory_event` 留痕。

packages/memory/__init__.py

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+"""记忆系统：分层记忆的读写、衰减、冲突消解。占位。"""