[FEATURE]: Keep the model when switching the agent

[MstKenway]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Is there any plan to bring the env var OPENCODE_AGENTS_SWITCH_SINGLE_MODEL back? Just as mentioned in #3028 (comment)

[zpin]

Would be great to have that back.