agent.*.model config is ignored - all agents use the top-level model

Description

Setting agent.<name>.model in mimocode.jsonc has no effect. All agents (build, plan, compose, explore, etc.) fall back to the top-level model setting regardless of per-agent config.

Config

{
  "$schema": "https://mimo.xiaomi.com/mimocode/config.json",
  "model": "standard",
  "model_groups": {
    "ultra": "xiaomi/mimo-v2.5-pro",
    "standard": "xiaomi/mimo-v2.5",
    "lite": "xiaomi/mimo-v2.5"
  },
  "agent": {
    "plan":    { "model": "ultra" },
    "build":   { "model": "standard" },
    "compose": { "model": "ultra" }
  }
}

Expected behavior

• agent.plan.model: "ultra" -> API calls use xiaomi/mimo-v2.5-pro
• agent.build.model: "standard" -> API calls use xiaomi/mimo-v2.5
• agent.compose.model: "ultra" -> API calls use xiaomi/mimo-v2.5-pro

Actual behavior

All agents use xiaomi/mimo-v2.5 (resolved from top-level model: "standard"). The per-agent model config is completely ignored.

Changing the build agent's model to a literal xiaomi/mimo-v2.5 caused ALL agents to switch to mimo-v2.5, further confirming the per-agent config is not applied independently.

Evidence from logs

Every API call in ~/.local/share/mimocode/log/ shows modelID=mimo-v2.5 regardless of agent name:

agent=build   -> modelID=mimo-v2.5  (correct, but by coincidence)
agent=plan    -> modelID=mimo-v2.5  (should be mimo-v2.5-pro)
agent=explore -> modelID=mimo-v2.5  (correct)

No mimo-v2.5-pro API calls appear anywhere in the logs, despite plan and compose being configured with "ultra".

Environment

• MiMoCode version: 0.1.14
• OS: Windows
• Config file: ~/.config/mimocode/mimocode.jsonc
• No project-level .mimocode/mimocode.json exists
• No custom agent .md files exist