fix subagent default model config

[Gucc111]

Summary

• parse agent.subagents.default and carry it into runtime config as an optional subagent baseline model
• apply the configured subagent model and caps when building subagent sessions
• preserve router/token-saver behavior by treating the setting as a baseline rather than a forced router override

Root Cause

The Web UI persisted the subagent default model under agent.subagents.default, but the backend parser and subagent fork path never read that field. Subagents therefore kept inheriting the parent agent model even when the UI showed a different configured model.

Validation

• pnpm exec tsc -p tsconfig.json --noEmit
• pnpm run build
• node --test --test-timeout 60000 dist/tests/agent/sub/SubAgentSession.spec.js
• Manual Gateway WebSocket validation against a running PilotDeck instance with main model and subagent default model set differently