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/rules/__init__.py

@@ -0,0 +1 @@
+"""规则引擎：决策表求值、硬约束校验、命中链输出。占位。"""