fix(agent-core-v2): degrade broken secondary-model pool entries to startup warnings

[sailist]

Related Issue

N/A — reported via internal feedback (no public issue).

Problem

If any entry in the [secondary_model.models] pool references a model alias that no longer exists in [models] (typical after a provider is deleted or logged out), the session-startup pre-flight threw a configuration error, so every session create/resume/fork failed — the user had to hand-edit config.toml before any session could be opened at all.

What changed

Secondary-model pool problems no longer block session startup, mirroring the v1 engine's long-standing advisory semantics:

• Effective pool view: pool entries that fail to resolve (and the reserved primary alias) are filtered out; a missing or broken default_model falls back to the first surviving entry; when nothing survives, subagents inherit the caller's model. The Agent/AgentSwarm tool descriptions and binding choices only see the surviving entries.
• Warnings instead of errors: the create/resume/fork pre-flight now logs pool issues instead of throwing, and the session-warnings surface (SDK getSessionWarnings, GET /sessions/{id}/warnings) reports each issue as a secondary-model-invalid warning, which the TUI renders when the session opens.
• Lazy backstop kept: a spawn that still binds a broken model — an explicit model request naming it, or a force pin — fails with the existing wrapped, actionable error naming the entry.
• Removed the session-scope validation sentinel service (nothing consumed it; its constructor failure only produced a sticky failed DI unit).
• Docs (en/zh) updated to the new semantics; changeset included.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (external PRs: the issue must have a maintainer's /approve).
• 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: 8ba05c6

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

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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commit: 8ba05c6

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[chatgpt-codex-connector]

Report an invalid default when the pool is empty

When [secondary_model.models] is present but empty, this early return produces no issues even if default_model is configured or required. Since the schema accepts an empty record, a configuration such as default_model = "provider/fast" followed by an empty models table silently makes subagents inherit the caller model, without the promised startup warning that the default is not a usable pool key. Validate the missing/unavailable default before returning for zero surviving aliases.

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