docs: add hooks research synthesis

[tkstang]

Summary

This PR adds a synthesized hooks research set for OAT's Codex and Claude Code hook compatibility work.

• add the GPT-5 Codex deep-research artifact for provider-agnostic hooks
• move the Opus deep-research artifact into .oat/repo/reference/research
• add a synthesis artifact reconciling the Opus and Codex reports, including agreements, contradictions, and consolidated recommendations

Why

The research now lives in the OAT reference research area and includes a single synthesis artifact that can drive follow-on docs, hook abstractions, and provider-adapter design work.

Validation

• pre-commit markdown formatting via oxfmt --write
• pre-push pnpm type-check
• pre-push pnpm lint
• pre-push pnpm format