feat: support Codex hooks engine (v0.114.0+) SessionStart/Stop events

[boxp]

Summary

• Add Codex hooks (v0.114.0+) SessionStart / Stop event support to ceeker hook handler
• Existing notify mechanism preserved as fallback for pre-v0.114.0 Codex versions
• README (en/ja) updated with Codex hooks configuration example using async execution

Why hooks support now

Codex rust-v0.114.0 introduced an experimental hooks engine with SessionStart and Stop hook events. This enables ceeker to receive session lifecycle events via stdin JSON (same mechanism as Claude Code hooks), providing a cleaner integration path compared to the notify argv-based approach.

Hooks vs notify fallback strategy

• Hooks (recommended): ~/.codex/hooks.json — stdin JSON payload with session_id, cwd, hook_event_name, last_assistant_message
• Notify (fallback): ~/.codex/config.toml notify array — argv-based JSON payload
• Both pathways share the same normalization pipeline (codex-event-fields, extract-codex-identity)
• The handler uses hook_event_name field (present in hooks payloads) as a new resolution source in resolve-codex-event

Async config choice

The README config examples use async: true so ceeker hooks run in the background and do not block the Codex agent loop. A note warns that async may not be supported in all versions and provides a sync fallback.

Changes

• handler.clj: Add SessionStart, Stop (PascalCase) to codex-event-fields; hook_event_name fallback in resolve-codex-event; session_start to SessionStart mapping in codex-type->event; last_assistant_message capture on Stop
• handler_test.clj: 7 new tests covering hooks SessionStart/Stop normalization, hook_event_name fallback, lifecycle E2E, snake_case mapping, notify regression
• README.md / README.ja.md: Codex hooks (recommended) + notify (fallback) sections

Tests

• Codex hooks SessionStart normalization: running, no message
• Codex hooks Stop normalization: completed, captures last_assistant_message
• Codex hooks Stop without message: falls back to "session ended"
• hook_event_name fallback resolution (no explicit CLI event-type)
• Full SessionStart to Stop lifecycle E2E with store verification
• session_start snake_case via type field mapping
• Notify pathway regression test

Residual risk / review points

• Codex hooks engine is experimental — config format may change in future releases
• async: true may produce a warning in current Codex versions (noted in README)
• Only SessionStart and Stop events are available; no Notification equivalent yet in Codex hooks
• ceeker hook handler is intentionally kept lightweight to minimize sync execution impact