refactor: extract command idempotency types

[chuanman2707]

Summary

• Extract command idempotency contract/type surfaces into command_idempotency/types.rs.
• Keep root command_idempotency.rs as the executor/runtime module with public re-exports for compatibility.
• Leave ARCH-14 Batch 3: Document command idempotency contract and assert outbox write core #147 planning/docs out of the PR.

Validation

• cargo test command_idempotency
• cargo clippy --all-targets -- -D warnings

Scope

• Code-only refactor for ARCH-15 Batch 3: Extract command idempotency types and constants #148; no behavior change intended.