fix: Codex CLI SessionStart additionalContext at top level (#505) (#508)

* fix: Codex CLI SessionStart additionalContext at top level (#505)

Codex CLI expects additionalContext at the top level of the hook output,
not nested under hookSpecificOutput. Flattened the Codex branch of
writeHookOutput so the injected instructions actually reach the agent.

Fixes #505

* Merge main; move #99 Codex @ponytail report assertion to top-level additionalContext (#505)

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Co-authored-by: Emeriko <dietrich.gebert@gmail.com>