chore: default model gpt-6.1-sol -> gpt-5.6-terra (#60)

Co-authored-by: host <host@test.local>

host-config.example.json

@@ -1 +1 @@
-{"_note":"复制成 host-config.json 再改；API key 绝不放这里，只走环境变量","approval":"deny","approval_policy":"never","codex_home":"","effort":"low","model":"gpt-6.1-sol","repo":"work/prod-repo","sandbox":"workspace-write","state":"host-state.json"}
+{"_note":"复制成 host-config.json 再改；API key 绝不放这里，只走环境变量","approval":"deny","approval_policy":"never","codex_home":"","effort":"low","model":"gpt-5.6-terra","repo":"work/prod-repo","sandbox":"workspace-write","state":"host-state.json"}