Memory flush, /compress, and session hygiene create AIAgent without
model=, falling back to the hardcoded default "anthropic/claude-opus-4.6".
This fails with a 400 error when the active provider is openai-codex
(Codex only accepts its own model names like gpt-5.1-codex-mini).
Add _resolve_gateway_model() that mirrors the env/config resolution
already used by _run_agent_sync, and wire it into all three temporary
agent creation sites.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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