feat(parser): migrate codex provider

Codex sessions have an append-only JSONL transcript plus a session_index.jsonl title sidecar. Moving Codex behind a concrete provider keeps that composite source identity and incremental append capability explicit at the provider boundary.

The provider preserves dated and archived discovery, live-over-archived lookup, shallow index watch planning, index-event classification, index-aware mtimes, source hashing, full parse output, and append parsing with full-parse fallback signals.

Author: mariusvniekerk