* feat(agents): per-agent model and generation settings
Let each custom agent choose its own model and sampling settings
(temperature, max_tokens) plus thinking / reasoning_effort defaults,
so agents sharing a model profile are no longer stuck with one shared
temperature and output length (#4336).
AgentConfig gains optional model_settings / thinking_enabled /
reasoning_effort (None = inherit). create_chat_model applies per-caller
model_overrides on top of the profile before the thinking/Codex
transforms; the lead agent resolves each knob with precedence
request > agent config > profile/default. The /api/agents create/update
routes persist the fields and reject an unknown model. The default lead
agent path is unchanged (no agent config -> overrides None). The agent
chat composer also stops force-overriding an agent's configured default
model with models[0].
* fix(agents): tri-state thinking control and default-model capability gating
The model-settings dialog seeded the thinking switch to false, so opening
it to tweak temperature and saving silently disabled thinking (the runtime
default is on) with no way back to inherit. It also hid the thinking /
reasoning controls whenever the agent inherited the global default model,
since `__default__` never resolved through `models.find`.
Give thinking an explicit Inherit / On / Off tri-state so an untouched save
is a no-op, and resolve `__default__` to the effective default (models[0])
for the capability check. Logic lives in the tested helpers module.
