Standardize Inkbox plugin behavior

[dimavrem22]

Summary

• Standardizes webhook dedup with signature-first verification, in-flight suppression, commit-after-dispatch, and rollback on parse/handler failure.
• Adds stable inbound SMS/iMessage event-id dedup tests.
• Brings Codex to the shared 18-tool coding-agent Inkbox tier by adding contact delete.
• Documents Realtime env knobs and late voice reply drop behavior.

Tests

• python3 -m pytest
• python3 -m pytest tests/test_gateway_dedup.py tests/test_gateway_inbound_media.py tests/test_tools.py

Notes

• Full failed-delivery recovery wakeups remain deferred to the follow-up delivery-failure PR set.