Extract command claim/replay helpers

[chuanman2707]

Summary

• Extract command idempotency claim/reclaim/replay helpers into command_idempotency/claim_replay.rs.
• Rewire non-atomic executor paths through the new module without changing replay/reclaim semantics.
• Keep execute_atomic transaction ownership in the executor and reuse only existing-row resolution.

Test Plan

• cargo clippy --manifest-path mhm/src-tauri/Cargo.toml --all-targets -- -D warnings
• cargo test --manifest-path mhm/src-tauri/Cargo.toml command_idempotency

Closes #150