Keep Codex 0.130 JSON output visible (#27)

Accept both legacy assistant_message and current agent_message item.completed payloads, and keep usage metadata flowing from turn.completed events.

Constraint: Codex CLI 0.130.0 emits item.type=agent_message and turn.completed usage while older versions used item_type=assistant_message and task_complete.

Rejected: Replacing the parser branch wholesale | the compatibility gap is narrow and the existing adapter shape already handles adjacent Codex events.

Confidence: high

Scope-risk: narrow

Directive: Preserve both old and new Codex JSON shapes when changing this adapter.

Tested: pnpm exec tsc --noEmit; parser regression script for assistant_message, agent_message, and turn.completed usage; pnpm build.

Not-tested: Live /api/convert run against Codex 0.130.0 in this branch.

Co-authored-by: Tack <170688654+tackcrypto1031@users.noreply.github.com>
Co-authored-by: OmX <omx@oh-my-codex.dev>