Subagents cannot easily run on a different model than the main agent

[imadcat]

Problem

Subagents spawned via the Agent / AgentSwarm tools always run on the same model as the main agent unless the operator discovers and enables the experimental secondary-model flag (KIMI_CODE_EXPERIMENTAL_SECONDARY_MODEL), which is off by default. Even with the flag on, the model parameter only accepts the symbolic choices primary / secondary — there is no way to point a single subagent at an arbitrary configured [models] alias, and agent profiles (model_preference) cannot name a concrete model either.

Proposal

1. Promote the secondary-model mechanism to default-on (opt out via KIMI_CODE_EXPERIMENTAL_SECONDARY_MODEL=0 or [experimental] secondary-model = false), keeping the legacy inherit-the-parent-model behavior available.
2. Let the Agent / AgentSwarm model parameter accept any configured [models] alias in addition to primary / secondary.
3. Let profile modelPreference / agent-file model_preference name a concrete alias, validated at spawn time.
4. Fail loudly on unknown aliases (error listing the valid choices) instead of silently inheriting the parent's model.

I have an implementation ready covering both engines (agent-core and agent-core-v2) with tests and docs — will open the PR right away.