[done] PR #114 で agent_accounts foundation + mention E2E landing。残 scope (別 PR):

- `agent` envelope field 廃止 + event log migration
- AuthorBadge user/agent 統合 + AI/dispatch chip 廃止
- create_post auto-provision + OWNER_HANDLE env
- mention chip click 挙動 wiring

clean 方針自体は維持、review 性で 1 段譲歩。