[FEATURE]: give task/subagent an optional config to choose what model it runs on

[modpotato]

Feature hasn't been suggested before.

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

Describe the enhancement you want to request

for an example arch (supervisor-ish):

plan mode

glad to hear you're happy with this implementation plan! lets begin

[tool call general subagent, prompt "write bla bla bla frontend" and uses model codenamed "webui-best" which maps to glm 4.7]

impl: https://github.com/modpotato/opencode/commit/22300b9056e81a9faf98dafdb46a22156b4eed8b

this is mainly for the "For net-new functionality, start with a design conversation" as it seems new enough a discussion, but otherwise ill just send the pr over :)