fix(server): capture Codex skill warnings during agent creation

[edvardchen]

Summary

• surface Codex skills/list validation failures as timeline errors instead of silently dropping them
• hydrate provider history immediately after createAgent so startup warnings are recorded for newly created Codex agents
• add regression coverage for both the provider warning mapping and the create-agent hydration path

Bug

When Codex returns startup warnings from skills/list such as invalid SKILL.md files, Paseo only exposed them through provider history. Resume and refresh flows hydrated that history, but the create-agent flow did not. As a result, a newly created Codex agent could start with a warning like Skipped loading 1 skill due to invalid SKILL.md file, yet the daemon timeline stayed empty and the desktop app showed nothing.

Repro

1. Create an invalid .codex/skills/*/SKILL.md.
2. Start the daemon and create a brand new Codex agent.
3. Observe that Codex reports the warning internally, but the new agent timeline in Paseo/desktop does not show the error.

Fix

The provider now turns skills/list.errors[] into timeline error items, and the create-agent path now calls hydrateTimelineFromProvider(snapshot.id) before forwarding the new agent snapshot. That makes the startup warning durable and visible to desktop on first creation, not only after resume/refresh.

Testing

• ./node_modules/.bin/vitest run packages/server/src/server/session.test.ts packages/server/src/server/agent/providers/codex-app-server-agent.test.ts --bail=1