TOMATO-61: Add idempotency-key deduplication in executor flow

[cracketus]

Summary

• add idempotency_key to ActionV1 for execution deduplication requests
• add deterministic idempotency cache config (IdempotencyConfig) with TTL and bounded retention
• integrate idempotency dedup checks into HardwareExecutor before dispatch
• skip duplicate requests with clear reason code (idempotency_duplicate) and note metadata
• persist idempotency tracking events (idempotency_stored, idempotency_hit) in executor_log.jsonl runtime events
• keep backward compatibility when no idempotency key is provided
• update Stage 5 runtime docs in README/AGENTS/PLANNED_FEATURES

Testing

• pytest -q --no-cov tests/contracts/test_action_v1.py tests/executor/test_idempotency.py tests/executor/test_hardware_executor.py tests/integration/test_24h_deterministic_run.py
• result: 30 passed

Closes #73