fix(overseer): hide tool-incompatible brain models (gpt-5.6-luna converse 400)

[heavygee]

Problem

Overseer talk (POST /api/overseer/converse) fails when active model is gpt-5.6-luna:

Function tools with reasoning_effort are not supported for gpt-5.6-luna in /v1/chat/completions. To use function tools, use /v1/responses or set reasoning_effort to 'none'.

Brain shows reachable; converse returns the soft "tool conversation (request error)" reply.

Ask

Overseer always uses chat/completions + function tools. Do not offer models that cannot run that path. Clear/reject an already-persisted incompatible active model.

Notes

Operator dogfood on /overseer 2026-09-07. Optional belt: send reasoning_effort: none when tools are present so recoverable reasoning models work — still keep the picker honest.