perf: parallelize new-session model discovery

[jonathanKingston]

Summary

• start model discovery while the new Codex thread is being created
• keep skill refresh and session-config construction ahead of both requests
• add a regression test that holds thread/start open and proves model/list is already in flight

Why

Neither result depends on the other, but newSession previously awaited thread/start before issuing model/list. Codex may refresh its model catalog in both requests, so slow or timing-out network refreshes were paid serially on the ACP session startup path.

In a real Codex 0.154.0 process inside Copse's production macOS sandbox, session/new took 6.33s and 6.53s with the serial requests. Running the same two requests concurrently reduced the measured session/new time to 3.18s. This removes roughly one refresh timeout without changing the returned session or model catalog.

Validation

• npm run typecheck
• npm run build
• npm test: 700 passed, 26 skipped
• focused CodexAcpClient suite: 106 passed
• authenticated real-Codex smoke: completed an ACP turn with end_turn