feat: support a configurable secondary model for subagents

[7Sageer]

Related Issue

No GitHub issue — the requirement comes from an internal design doc ("主子 Agent 模型定义", Moonshot Feishu wiki). The problem is explained below.

Problem

Subagents always inherit the calling agent's model. Users who want to save cost or capacity on a strong primary model have no way to run subagents on a cheaper secondary model: there is no configuration surface for a per-purpose model, and the main agent cannot choose a different model per spawn. Fork agents must keep using the parent's model (they share context), so the choice only applies to fresh spawns.

What changed

• New [subagent] model / effort fields in config.toml, with KIMI_SUBAGENT_MODEL / KIMI_SUBAGENT_EFFORT env overrides (env wins and is never persisted). The value is any configured model alias, so the secondary model is not limited to Kimi models.
• When configured, Agent / AgentSwarm spawns bind the secondary model (and optional effort) by default; both tools gain a model parameter ("subagent" | "primary") and advertise the two concrete aliases in their descriptions so the main agent can pick per spawn. When nothing is configured, spawning behavior is byte-for-byte unchanged (subagents inherit the caller's model).
• Resumed subagents now keep the model recorded in their own wire journal instead of being realigned to the parent agent's current model — the old invariant contradicts per-subagent models. Resuming a subagent requires the model it was created with to still be configured.
• Spawn failures caused by an invalid secondary model alias are wrapped with a pointer to the config knob; other creation failures pass through untouched.
• Fork agents, /init, main-model resolution, and legacy agent-core are unchanged. No automatic default secondary model is wired in this phase (explicit configuration only).
• Docs updated (config-files, env-vars, tools reference; en + zh). Tests were added to existing test files only. A changeset is included.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: d2ce676

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@moonshot-ai/agent-core-v2	Minor
@moonshot-ai/kimi-code	Minor
@moonshot-ai/kap-server	Patch
@moonshot-ai/klient	Patch

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